The individual plan for your horse would depend on a couple things. The biggest are:
What kind of shape is he in now?
What kind of terrain do you have to work with?
For the barrel horse my niece rides, we're working him up to 6-8 miles one day, 3-5 miles the next, and alternate. He gets day 7 off. Half of this is long trotting, and half is a relaxed lope.
When conditioning any of my horses, I like to find hills to ride on. I trot straight up and down, and across the face. The straight up and down is especially useful for building the hindquarter, and the cross-face is good for lateral movement.
I hope that made sense. Just remember to build up to whatever you do.