I lucked out last year as my two year old colt conditioned himself quite nicely given plenty of turnout. He ran, and ran, and ran, and ran himself right into fitness!
Our paddocks are dry lots so my boys get plenty of grass hay three times a day (the colt got practically free-choice hay with all the calories he was burning), a complete feed, and in his case 2.5 cups of soaked beet pulp twice a day. I believe in the natural approach to keeping my horses which means plenty of roughage in front of them multiple times a day, lots of turnout and as much social interaction as is appropriate given the individuals involved. My boys, for instance, need to be separated at feeding time so Kody doesn't terrorize Turbo but they now spend most of the day out together. Unfortunately that does result in plenty of bite marks and little bald patches on my halter horse but on the other hand they get great muscle tone and lots of cardio.
This year he is three so I will be fitting him up more formally. I don't have a roundpen so he'll be free-lunged for increasing amounts of time at trot and canter, worked over cavaletti poles and will get ponied off the back of my mature horse's cart for longer straight-line workouts that won't be as hard on his joints as lunging would be. That's the only way I've ever found to get portly Mr. Kody in shape- DRIVE! For a long time. Frequently. With engagement and energy.
When Kody retired he looked wonderfully toned and had flipped his upside-down neck almost completely right-side up through proper work. One of my greatest sorrows was watching that fade while he was laid up from his back injury.
It's been two years and I'm going to start him back to work slowly and carefully in hopes of conditioning him but frankly I don't know if I'll ever see his waistline again.
Turbo, however, should look better and better over the next few years as he's already trim and now we'll tone that. There is nothing like proper driving work to enhance the topline and chest of a mature horse.
Leia