You should not need to diet at all. Stomach shrinks. Big bodies expend major energy on the track relative. And it curbs appetite for trash when you leave grease filled tacos on track next day.
LOL...some of my friends used to eat McD's and pizza before workouts back in high school, but they quickly learned better.
The more you train, the more you crave healthy foods. And of course you have to eat a lot more fruits and vegetables to be full than you do with doughnuts or Taco Bell.
Of course, a varied diet is important - whole grains, low-fat dairy and lean meat in addition to produce.