Sprints are really good for burning fat/building muscle.
A good method I use (that helps the time go by fast as well) is to sprint for 30 seconds, then walk for 30 seconds - over and over for a couple of miles. Even if you just do this for 10 minutes, you'll get a heck of a workout. *You can also do 15 second intervals if you want - but the goal is to walk the same amount that you sprint.
Be careful with the knees though - as with any sort of running/biking. If you can avoid pavement, that'll be much easier on you in the long run.