I think we all are different. I have no probs with working out two hours. I usually do one hour after i take my husband to the train at 500am and another when my kids watch tv or take a nap or before i go to bed, which works for me. I work out at home because i have the flexability with 4 kids and a hubby and working part time I can not always get out for a run but I will buy a treadmill for that. We all have to find what works for us somew days I only do wgts b/c thats what I feel like. I agree with Cathe any time pro athletes do 4 hours a day it is safe. In high school I had 3 hour bball practice and the next day 3 hour soccer in the hot sun. So now at the age of 28 I choose what I want.