I strongly reccomend a heart rate monitor. A little reading will help you find your anerobic threshold (AT), which is below your max. There is a great pdf file at
www.lifetimefitness.com (the gym I go to - click on health info,then health tips and then heart rate training) that helps you see the different zones and figure out your own AT. Once you do that, spending time in a zone 2 and 3 will help your body learn how to use fat instead of just carbs when you are working out. Training your body that way is much more effective than all out work - all the time.
But balance in everything is important. The sprints do help you build muscle, as muscle burns a lot more fat just to keep the muscles alive. And you can do them on a bike or eliptical trainer. I prefer a spin bike. Plus, a monitor will help you work out to increase your AT and max heart rate.