I'm on a low carb diet at the moment – get plenty to eat, but struggle a bit with the energy so I usually fill my waters with the flavor packs that are the energy boosters, or an occasional pop just to keep from getting a lack of caffeine headache. Low carb makes it really easy to drop the weight, and as for exercise I'd say core workouts. It's always good to be strong in your core, arms, and legs, so I suggest separating the work throughout there. Also, meal planning helps a whole bunch. :)
Biggest thing about diet is remember to eat!!!! Food is fuel so plan what you will need and how you will get it.
Packing at least your own snacks for the day ensures you stay on track.
Body builders diet is identical to a dietetic. Five small meals a day with most of the calories and sugars (hopefully natural based sugar) before 3:00 pm. Fat is not the enemy, processed sugars are.
Best rule is snack or graze every three hours, include proteins and good carbs. Your body is a machine. If you want to keep it running properly you need to fuel it
As far as exercise goes there is no better workout for both you and you horse than good proper flat work
Find a good dressage trainer who will push your flat skills. All of our riders look like they just finished a marathon after one of our trainers dressage lessons lol
Any core or cardio will be helpful. Pick something you enjoy so you are more likely to do it. Riding proper flat is still one of the best exercises