Which way are you leaning? Too much revision or too much rest? If too much revision - use exercise as your rest. Set a timer for 45 minutes or 1 hour for revision and when it goes off, get up and move about for 5 or 10 minutes. Every 3 hours, take 1 hour off and get some more intense exercise. This will help your brain to process the work you have been doing and help you learn more effectively.
If too much rest, then use a timer to stop you resting and get back to work. Every time you find yourself not revising, set the timer for 5 minutes and when it goes off - get back to work.