depends if you have 15 lbs to lose. if you're talking about beefing down, probably don't do it, but if you're talking about losing a few pounds off your gut, then yeah, most definitely do it. i've heard some formulas of something like 3 seconds/mile/pound, which seems reasonable if you've got the weight to lose. However, that's a long term thing--remember that it's always a shock to the system to lose fat reserves, so for a while it may seem like it's not really getting you any faster even though you're no longer eating twinkie sundaes for breakfast. And if you seriously have 15 (0r even 10) pounds to lose with those times, that is seriously very impressive.