(pic from last year, he's even fatter now)
IF you do find yourself in this predicament here are the steps to follow:
1) Put the chubby donkey in a pen or dry lot with a horse.
2) Feed the donkey & his horse with slow feeder hay nets, you can reduce the portions of hay but maintain a "grazing" type of feeding.
3) Once the donkey has formed an attachment to "his" horse you can start to exercise your ass... simply remove the horse from the pen for periods of time. Better yet, remove his horse from the donkey's sight. (example, take the horse into the barn to groom)
Donkey will pace, run and buck in the pen.
For advanced donkeys you could also take the donkey into an arena and ride the horse, as you trot and lope circles the donkey will alternate between standing around and madly chasing "his" horse.