Was going to comment, but I'll make this an answer - its the bike, not you. No, I really do mean its the bike.....

The bike as a fixie with 48/19 gearing is suitable for a cycle fit 20 something year old hipster with great knees now and a good health insurance plan for future orthopedic consultations. Installing the freewheel helps make it more versatile, but 48/19 is still a big gear for a novice rider to push. You should be riding at at least about 13mph on that gear - any less and your cadence is too low and will cause the muscle burn you feel and make the bike feel 'sluggish'.

On that bike, an average, no-cycle fit 39yo will almost certainly not enjoy cycling and give up. You base fitness will help a little to start with when riding, but you are not cycle fit. You legs are not used to the motion and you use different muscles. The body needs time to adapt to the new exercise pattern, yet that bike does not allow you to do so progressively.

You may be able to improve the bike by improving the fit (A professional fitter can help a lot here), but ultimately the bike is not the right bike for the task you have purchased it.

Was going to comment, but I'll make this an answer - it's the bike, not you. No, I really do mean it's the bike.....

The bike as a fixie with 48/19 gearing is suitable for a cycle fit 20-something year old hipster with great knees now and a good health insurance plan for future orthopedic consultations. Installing the freewheel helps make it more versatile, but 48/19 is still a big gear for a novice rider to push. You should be riding at at least about 13mph on that gear - any less and your cadence is too low and will cause the muscle burn you feel and make the bike feel 'sluggish'.

On that bike, an average, no-cycle fit 39yo will almost certainly not enjoy cycling and give up. Your base fitness will help a little to start with when riding, but you are not cycle fit. Your legs are not used to the motion and you use different muscles. The body needs time to adapt to the new exercise pattern, yet that bike does not allow you to do so progressively.

You may be able to improve the bike by improving the fit (a professional fitter can help a lot here), but ultimately the bike is not the right bike for the task you have purchased it for.

Was going to comment, but I'll make this an answer - its the bike, not you. No, I really do mean its the bike.....

The bike is a fixie with 48/18 gearing - suitable for a cycle fit 20 something year old hipster with great knees now and a good health insurance plan for future orthopedic consultations.

On that bike, an average, no-cycle fit 39yo will almost certainly not enjoy cycling and give up. You base fitness will help a little to start with when riding, but you are not cycle fit. You legs are not used to the motion and you use different muscles. The body needs time to adapt to the new exercise pattern, yet that bike does not allow you to do so progressively.

You may be able to improve the bike by improving the fit (A professional fitter can help a lot here), but ultimately the bike is not the right bike for the task you have purchased it.

Was going to comment, but I'll make this an answer - its the bike, not you. No, I really do mean its the bike.....

The bike as a fixie with 48/19 gearing is suitable for a cycle fit 20 something year old hipster with great knees now and a good health insurance plan for future orthopedic consultations. Installing the freewheel helps make it more versatile, but 48/19 is still a big gear for a novice rider to push. You should be riding at at least about 13mph on that gear - any less and your cadence is too low and will cause the muscle burn you feel and make the bike feel 'sluggish'.

On that bike, an average, no-cycle fit 39yo will almost certainly not enjoy cycling and give up. You base fitness will help a little to start with when riding, but you are not cycle fit. You legs are not used to the motion and you use different muscles. The body needs time to adapt to the new exercise pattern, yet that bike does not allow you to do so progressively.

You may be able to improve the bike by improving the fit (A professional fitter can help a lot here), but ultimately the bike is not the right bike for the task you have purchased it.