I was a complete couch potato 6 months ago, I then started a daily (90-120min) cardio routine. Early on I tried running but it hurt my knees too much so I left it alone. Recently I set a goal of doing a tri sprint in the spring so the other day I hit the treadmill as a test...I managed 40 solid mnutes at 6.5mph after nothing for prior to that. I will say my muscles have been pretty sore the days since, but I survived so I assume it can be done.