Return to Answer

Which suggests it was around 40 miles (~64 km). Thanks to Valorum’s helpful annotation, we can see that this is about 36 miles (~58 km), (click all images to embiggen):

The Tale of Years in Appendix B and some of the Return of the King puts it to around a 3/4 day March from Minas Tirith, possibly quicker if it’s a small party instead of the Host of the West or a healthy group rather than Frodo and Sam

9 Gandalf reaches Minas Tirith. Faramir leaves Henneth Annûn. Aragorn sets out from Erech and comes to Calembel. At dusk Frodo reaches the Morgul-road. Théoden comes to Dunharrow. Darkness begins to flow out of Mordor.
10 The Dawnless Day. The Muster of Rohan: the Rohirrim ride from Harrowdale. Faramir rescued by Gandalf outside the gates of the City. Aragorn crosses Ringló. An army from the Morannon takes Cair Andros and passes into Anórien. Frodo passes the Cross-roads, and sees the Morgul-host set forth.
11 Gollum visits Shelob, but seeing Frodo asleep nearly repents. Denethor sends Faramir to Osgiliath. Aragorn reaches Linhir and crosses into Lebennin. Eastern Rohan is invaded from the north. First assault on Lórien.
12 Gollum leads Frodo into Shelob's lair. Faramir retreats to the Causeway Forts. Théoden camps under Minrimmon. Aragorn drives the enemy towards Pelargir. The Ents defeat the invaders of Rohan.

<!— —>

The vanguard passed on through the ruins of Old Gondor, and over the wide River, and on up the long straight road that in the high days had been made to run from the fair Tower of the Sun to the tall Tower of the Moon, which now was Minas Morgul in its accursed vale. Five miles beyond Osgiliath they halted, ending their first day's march.
_{The Return of the King - Book 5, Chapter 10: The Black Gate Opens}

_{note, a small addendum will be made to the below to fix the starting point from Osgiliath to Minas Tirith, we know this is around a days March, and ~ 12 miles.}

Which suggests it was around 40 miles (~64 km). Thanks to Valorum’s helpful annotation, we can see that this is about 36 miles (~58 km), (click all images to embiggen):

The Tale of Years in Appendix B and some of the Return of the King puts it to around a 3/4 day March from Minas Tirith, possibly quicker if it’s a small party instead of the Host of the West or a healthy group rather than Frodo and Sam

9 Gandalf reaches Minas Tirith. Faramir leaves Henneth Annûn. Aragorn sets out from Erech and comes to Calembel. At dusk Frodo reaches the Morgul-road. Théoden comes to Dunharrow. Darkness begins to flow out of Mordor.
10 The Dawnless Day. The Muster of Rohan: the Rohirrim ride from Harrowdale. Faramir rescued by Gandalf outside the gates of the City. Aragorn crosses Ringló. An army from the Morannon takes Cair Andros and passes into Anórien. Frodo passes the Cross-roads, and sees the Morgul-host set forth.
11 Gollum visits Shelob, but seeing Frodo asleep nearly repents. Denethor sends Faramir to Osgiliath. Aragorn reaches Linhir and crosses into Lebennin. Eastern Rohan is invaded from the north. First assault on Lórien.
12 Gollum leads Frodo into Shelob's lair. Faramir retreats to the Causeway Forts. Théoden camps under Minrimmon. Aragorn drives the enemy towards Pelargir. The Ents defeat the invaders of Rohan.

<!— —>

The vanguard passed on through the ruins of Old Gondor, and over the wide River, and on up the long straight road that in the high days had been made to run from the fair Tower of the Sun to the tall Tower of the Moon, which now was Minas Morgul in its accursed vale. Five miles beyond Osgiliath they halted, ending their first day's march.
The Return of the King - Book 5, Chapter 10: The Black Gate Opens

As highlighted by @Valorum in the comments:

This annotated map of Middle-earth by Tolkien gives us an insight into distances ¹

Which suggest it was around 40miles, thanks to Valorum’s helpful annotation, we can see that this is about 36 miles, (click all images to embiggen):

The Tale of Years in Appendix B and some of the Return of the King puts it to around a 3/4 day March from Minas Tirith, possibly quicker if it’s a small party instead of the Host of the West or a healthy group rather than Frodo and Sam

9 Gandalf reaches Minas Tirith. Faramir leaves Henneth Annûn. Aragorn sets out from Erech and comes to Calembel. At dusk Frodo reaches the Morgul-road. Théoden comes to Dunharrow. Darkness begins to flow out of Mordor.
10 The Dawnless Day. The Muster of Rohan: the Rohirrim ride from Harrowdale. Faramir rescued by Gandalf outside the gates of the City. Aragorn crosses Ringló. An army from the Morannon takes Cair Andros and passes into Anórien. Frodo passes the Cross-roads, and sees the Morgul-host set forth.
11 Gollum visits Shelob, but seeing Frodo asleep nearly repents. Denethor sends Faramir to Osgiliath. Aragorn reaches Linhir and crosses into Lebennin. Eastern Rohan is invaded from the north. First assault on Lórien.
12 Gollum leads Frodo into Shelob's lair. Faramir retreats to the Causeway Forts. Théoden camps under Minrimmon. Aragorn drives the enemy towards Pelargir. The Ents defeat the invaders of Rohan.

<!— —>

The vanguard passed on through the ruins of Old Gondor, and over the wide River, and on up the long straight road that in the high days had been made to run from the fair Tower of the Sun to the tall Tower of the Moon, which now was Minas Morgul in its accursed vale. Five miles beyond Osgiliath they halted, ending their first day's march.
_{The Return of the King - Book 5, Chapter 10: The Black Gate Opens}

¹ This was found annotated in one of Pauline Baynes’ copies of the LotR annotated by Tolkien himself and Baynes.

As highlighted by @Valorum in the comments:

This annotated map of Middle-earth by Tolkien gives us an insight into distances ¹

Which suggests it was around 40 miles (~64 km). Thanks to Valorum’s helpful annotation, we can see that this is about 36 miles (~58 km), (click all images to embiggen):

The Tale of Years in Appendix B and some of the Return of the King puts it to around a 3/4 day March from Minas Tirith, possibly quicker if it’s a small party instead of the Host of the West or a healthy group rather than Frodo and Sam

9 Gandalf reaches Minas Tirith. Faramir leaves Henneth Annûn. Aragorn sets out from Erech and comes to Calembel. At dusk Frodo reaches the Morgul-road. Théoden comes to Dunharrow. Darkness begins to flow out of Mordor.
10 The Dawnless Day. The Muster of Rohan: the Rohirrim ride from Harrowdale. Faramir rescued by Gandalf outside the gates of the City. Aragorn crosses Ringló. An army from the Morannon takes Cair Andros and passes into Anórien. Frodo passes the Cross-roads, and sees the Morgul-host set forth.
11 Gollum visits Shelob, but seeing Frodo asleep nearly repents. Denethor sends Faramir to Osgiliath. Aragorn reaches Linhir and crosses into Lebennin. Eastern Rohan is invaded from the north. First assault on Lórien.
12 Gollum leads Frodo into Shelob's lair. Faramir retreats to the Causeway Forts. Théoden camps under Minrimmon. Aragorn drives the enemy towards Pelargir. The Ents defeat the invaders of Rohan.

<!— —>

The vanguard passed on through the ruins of Old Gondor, and over the wide River, and on up the long straight road that in the high days had been made to run from the fair Tower of the Sun to the tall Tower of the Moon, which now was Minas Morgul in its accursed vale. Five miles beyond Osgiliath they halted, ending their first day's march.
_{The Return of the King - Book 5, Chapter 10: The Black Gate Opens}

¹ This was found annotated in one of Pauline Baynes’ copies of the LotR annotated by Tolkien himself and Baynes.

As highlighted by @Valorum in the comments:

This annotated map of Middle-earth by Tolkien gives us an insight into distances ¹

Which suggest it was around 40km, thanks to Valorum’s helpful annotation, we can see that this is about 36 miles, (click all images to embiggen):

The Tale of Years in Appendix B and some of the Return of the King puts it to around a 3/4 day March from Minas Tirith, possibly quicker if it’s a small party instead of the Host of the West or a healthy group rather than Frodo and Sam

9 Gandalf reaches Minas Tirith. Faramir leaves Henneth Annûn. Aragorn sets out from Erech and comes to Calembel. At dusk Frodo reaches the Morgul-road. Théoden comes to Dunharrow. Darkness begins to flow out of Mordor.
10 The Dawnless Day. The Muster of Rohan: the Rohirrim ride from Harrowdale. Faramir rescued by Gandalf outside the gates of the City. Aragorn crosses Ringló. An army from the Morannon takes Cair Andros and passes into Anórien. Frodo passes the Cross-roads, and sees the Morgul-host set forth.
11 Gollum visits Shelob, but seeing Frodo asleep nearly repents. Denethor sends Faramir to Osgiliath. Aragorn reaches Linhir and crosses into Lebennin. Eastern Rohan is invaded from the north. First assault on Lórien.
12 Gollum leads Frodo into Shelob's lair. Faramir retreats to the Causeway Forts. Théoden camps under Minrimmon. Aragorn drives the enemy towards Pelargir. The Ents defeat the invaders of Rohan.

<!— —>

The vanguard passed on through the ruins of Old Gondor, and over the wide River, and on up the long straight road that in the high days had been made to run from the fair Tower of the Sun to the tall Tower of the Moon, which now was Minas Morgul in its accursed vale. Five miles beyond Osgiliath they halted, ending their first day's march.
_{The Return of the King - Book 5, Chapter 10: The Black Gate Opens}

¹ This was found annotated in one of Pauline Baynes’ copies of the LotR annotated by Tolkien himself and Baynes.

As highlighted by @Valorum in the comments:

This annotated map of Middle-earth by Tolkien gives us an insight into distances ¹

Which suggest it was around 40miles, thanks to Valorum’s helpful annotation, we can see that this is about 36 miles, (click all images to embiggen):

The Tale of Years in Appendix B and some of the Return of the King puts it to around a 3/4 day March from Minas Tirith, possibly quicker if it’s a small party instead of the Host of the West or a healthy group rather than Frodo and Sam

9 Gandalf reaches Minas Tirith. Faramir leaves Henneth Annûn. Aragorn sets out from Erech and comes to Calembel. At dusk Frodo reaches the Morgul-road. Théoden comes to Dunharrow. Darkness begins to flow out of Mordor.
10 The Dawnless Day. The Muster of Rohan: the Rohirrim ride from Harrowdale. Faramir rescued by Gandalf outside the gates of the City. Aragorn crosses Ringló. An army from the Morannon takes Cair Andros and passes into Anórien. Frodo passes the Cross-roads, and sees the Morgul-host set forth.
11 Gollum visits Shelob, but seeing Frodo asleep nearly repents. Denethor sends Faramir to Osgiliath. Aragorn reaches Linhir and crosses into Lebennin. Eastern Rohan is invaded from the north. First assault on Lórien.
12 Gollum leads Frodo into Shelob's lair. Faramir retreats to the Causeway Forts. Théoden camps under Minrimmon. Aragorn drives the enemy towards Pelargir. The Ents defeat the invaders of Rohan.

<!— —>

The vanguard passed on through the ruins of Old Gondor, and over the wide River, and on up the long straight road that in the high days had been made to run from the fair Tower of the Sun to the tall Tower of the Moon, which now was Minas Morgul in its accursed vale. Five miles beyond Osgiliath they halted, ending their first day's march.
_{The Return of the King - Book 5, Chapter 10: The Black Gate Opens}

¹ This was found annotated in one of Pauline Baynes’ copies of the LotR annotated by Tolkien himself and Baynes.