I registered for the American Lung Associations Trek Across Maine. It is a 180 mile cycling trek in 3 days. I had to raise $500.00 and asked a friend to do it with me. I was committed to it and needed to train for it. It has kept me burning between 3000 and 6000 calories per week.
There is a book called "The Heart Rate Monitor Book For Cyclists" by Sally Reed and Sally Edwards. i swear by it. I would go nuts trying to stay on the bike for more than 20 minutes. This way the time flies