One data point:

Teddy Roosevelt got up and rode 100 miles, from sunrise to sunset, at 51 years old, after receiving so many complaints from army cavalrymen that had to ride 25 miles a day for training!

Some data points:

Teddy Roosevelt got up and rode 100 miles, from sunrise to sunset, at 51 years old, after receiving so many complaints from army cavalrymen that had to ride 25 miles a day for training!

Bud and Temple Abernathy - Rode 4,500 miles from New York to San Francisco in 62 days in 1911. They were aged 11 and 7 and traveled without adult supervision. == 72.5mi/day (and allegedly w/o buying new horses - but also, lighter weight than full grown adults).