No problem, I've just released another small update (3.4) that doubles the number of calories burned for all exercise actions. Solo jogging & swimming burn the most, so go for those if you're looking to slim down your player character.
You can unlock the Dizzy's addiction cure via the level 19 'Dizzy's Detox' upgrade. Once unlocked take the girl on a date to the doughnut shop and you will find a new option available.
As you found out, addicted girls will often demand that you go to Dizzy's with them. They'll also passively gain weight in the background from all the doughnuts they're eating when you're not around. If your shop is doing well & the town is starting to get fat, not only will many of the new girls you encounter be bigger, but they may also be addicted already.