if you're using it for a warm up, bike's ok as you don't have to endure it too long. it's also handy if you've eaten fairly close to a work out compared to running (i have to leave a gap of 3 hours after eating before i go for a run whereas i can get away with 2 hours if i'm going cycling). if you're using it to lose calories, go for running, cross training or rowing. better still, buy yourself a bicycle and get outside with it; much more fun - cycle to the gym, why not.