Intervals will burn fat by raising metabolism. Good thing. HOWEVER, endurance rides (heart rate at ~70% to 75%) teach your body to burn fat preferentially to sugar. Burning fat takes more work because your body must go through the Krebs cycle. Unfit people tend to be sugar-burners & then they bonk/quit. I've read that people who are endurance athletes burn more fat and the fat burning rate seems to increase proportionally to the amount of endurance fitness they've attained.