I try to work out about 2-3 times a week at the local gym. I usually do some cardio and some upper body for about an hour. I recently got back into riding for a couple reasons.

1) More fun to ride outside and actually get somewhere than ride on a stationary bike for 30 minutes and get nowhere

2) Exercise, I get a good lower body workout and cardio at the same time

But I have noticed my legs tend to get worn out before the rest of my body does... standing up while climbing hills and riding 15+ miles offroad will make my legs feel like rubber by the end of the ride. What can I do to condition my legs to be able to ride longer without fatigue. Squats, leg machines, and so on? I want to be able to ride longer, faster, and harder without getting burned out in a hour. What do you all do?