I've read that the idea of "fat-burning exercise" is largely a myth, it's total amount that counts. I found a decent link here:
http://www.hornetjuice.com/fat-burning-myths.html
"Yes, it is true that you burn a greater percentage of fat during low intensity exercise (actually you burn the greatest percentage of fat during sleep which is the ultimate in low intensity activity). However, the total number of calories burned is less so the absolute number of fat calories burned is less."
Varying intensity is good for training in other ways, though, for building strength or speed, or working on technique.
Edited to add: sorry, I read too fast. You didn't ask about fat, but about total calories. I think the principle is still the same, the harder you work, the more muscle you build and the more calories you burn.
Last edited by lph; 06-26-2012 at 11:02 AM.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett