For muscling and such, long trot 20 minutes each direction each day :) We do that with our halter horses and it really builds everything.
Be sure to stand with a triangle shape with you and your horse. The longe line to the horse's face, whip from the hind end back to you. Drive your horse forward with vocal and the whip, if needed. You may have to chase him some around the arena. Longing is definitely a learned thing, but it's not difficult to teach.
Jessica Fox & Shez Kinda Impressive
2009 APHA World Top Five SPB Longe Line
2009 ICPHA High Point Yearling Longe Line.