I use the treadmill in the winter when it's too dark or nasty outside for riding. I can't run either, so I've got a special treadmill designed for hikers - it has a 25% grade. So I can just slowly slog uphill for 90 minutes, gain 3,000 feet elevation and burn 700-1000 calories, depending on my speed (which is usually pretty slow - 2.5 mph) I find this good training for my spring hiking trips. Going fast without the incline doesn't get my heart rate up at all without jogging, so the incline is the way to go for me.