Actually, it's not a sin to indulge urself in chocolate, ice cream or candy, once in a while (chocolate in its purest form is a champion antioxidant, it helps rid the body of free radicals). As long as whatever u eat, goes out again of the body (by way of exercizing), it's alrite. There's no point in being too strict, or too lenient - just nice would be just fine. So there'll be a check and balance.
The problem starts when one eats too much junk food but does no exercize (or not enough exercizes). Thus, all the bad elements stay inside the body and that's when one needs to worry. One should try to exercize at least half an hour, daily (half an hour is the minimum). If u can't, try to make it at least thrice a week.
Yeah, i know the cravings for food can drive one crazy... but if u can managed 17 days w/out foods, then it's just a matter of manipulating ur mind to bend to ur wishes. Even i may not be able to make it up to 17 days!
Tell urself this one thing - u are allowed to run amok for foods, IF u can manage to exercize at least half and hour (1 hour is better!), daily. Now, that seems to be a good deal, isn't it?