I have two different horses and one was embarrassingly fat:
http://s9.postimage.org/upfuppd1b/003_3.jpg I worked with her and exercised her all spring;
http://s14.postimage.org/wwr2x8qlt/IMG_3175.jpg
and by the time the shows were here when I barrel raced her and showed her, here's her and now I got the fence working again so she won't be sneaking an y more hay now!
http://s9.postimage.org/x75e27133/017_2.jpg
http://s2.postimage.org/74nyvks6h/100_3377.jpg
Now here's when I bought Éowyn, she was a bit overweight:
http://s1.postimage.org/cbypw4ddb/mini_3.jpg
http://s13.postimage.org/vuycutvp3/mini_4.jpg
And here's her summer 2012 after almost a year with her (right now shes too fluffly to take a demonstrative pic
)
http://s9.postimage.org/4uacv3wan/Eowyn_14.jpg
http://s1.postimage.org/4krn8f4xb/009_3.jpg
What I did, what cut there feeding schedules; For Peanut (my big horse) I brought her in the stall during the day (the summer days were really hot too) and I took her out every few hours for exercise. For the first few times I just rode her at a walk for about a half hour to an hour. She wasn't nearly in shape and I didn't want to overwork her. Later on I started increasing her work and rode her at a walk, gave a break for a couple of hours, then took her back out and lunged her for 1 minute each way. Later the lunging increased more until I lunged her 5 minutes each way or more. When she was getting alot better and in shape, I brought her to the arena with her barrel saddle on and walked her around the arena and then trotted. Pretty soon she started loping and the more we went around she got faster and faster. Finally she was ready to barrel race again and we won 4th place!
With Éowyn, obviously I can't ride her but I did sort of the same. She doesn't have much grass in her pasture, so I let her out in the yard and she mows our lawn. So during the night shes in her pasture, and in the day I take her out. Once in a while I'll take her on walks and lunge her and she's in pretty good shape. =)
So I hope this helped at least a little! Good luck! =)