Curious if anyone else has this problem.

Under doctor's orders I have been walking everyday since February. This is to keep my legs strong and to help regain some balance that I seem to have lost with these never ending episodes of vertigo.

I have been walking the same distance, speed, terrain etc. and am up to an hour a day for the last 2 months. These are somewhat flat surfaces...not hills etc. I can not seem to pass this distance. I am not able to go further...In fact I noticed once the heat started my legs are screaming...the muscles do NOT like this exercise. Should I not be use to the distance etc. after 2 months? Anyone else have this problem?