I'm with Kitty on this one. You are probably overdoing it. Pace yourself and try to gage how your legs are doing. It is much harder to do that in a pool. Like Kitty mentioned, being in water can fool you. I break up my exercises because I fatigue quickly. Some days are better than others. Some days, I can only do 5 minutes. Other days, I can do 20 to 30 minutes of yoga (not walking or more strenuous exercises). Most of the time, I try to do 5 to 10 minutes in the morning and 5 to 10 minutes at night.

I hope this helps.