the 1st 20 min (give or take) of any run will burn what sugar you have stored up in your muscles.
then it will move on to the next source, depending on how you eat this could be ketones or protein.
jogging or keeping your HR around 65% will burn fat, but you wont burn shit for cals unless you do it for an hour. its a fucking waste of time.
intervals give you the best of both, and help build lean muscle, and where your done you have a better after burn.
i suggest reading Ready, Set, Go! Synergy Fitness. especially if you’re getting close to 30.
edit:
actually heres some info about the Sprint 8 program (from the book mentioned above)