Here are my two cents about getting started with endurance:
1) Volunter for at least one ride - definitely work some of that time at the vet check. Ask questions. There's a lot you can learn sitting with the vet and other volunteers (usually riders themselves) during the course of a 12 hour ride during which your job is mostly to sit, talk, and wait for a rider to show up. Observe and take in as much as possible about what's going on at camp, the vet check, ride meetings, starting the race, holds, and what riders are saying about the ride. You can learn more from one ride than a year of riding yourself.
2) Get your horse into fit and regular riding shape (you look like you're already there).
3) Get GPS that will tell you how far you've gone and how fast you're going (I have a Garmin Venture HC, about $110). This has been useful in riding in general - I log my arena miles just like my trail miles, and it sure is helpful on a ride to know how far you've gone and how far you have left to go! You'll also need a stethoscope. Don't worry about a heart rate monitor unless you really start getting into endurance. I still don't have one and don't feel any need for one, unless I start competing at the higher distances. Finally, you'll need a watch that shows the seconds.
4) Practice taking your horse's pulse with the stethoscope right behind his front left leg. There's plenty of websites to help you with this. When you start, time how many beats there are in 15 seconds and multiply by 4.
5) Get a good feel for a "working trot" - not a slow job, but not extended either. For most horses, this is about 8-9 mph (yours might be different because if his shape - I'm not familiar with the breed).
6) Take your horse out on a 4-5 mile ride and try to maintain a working jog most of the time. Keep cantering to a minimum, and walk where you need to. Cantering takes a LOT more out of a horse - that's why a good trot is the "working" gait of a horse. Walk the last 10 minutes/last couple hundred yards (figure out how far it takes your horse to walk 10 minutes).
7) Taking the pulse. When you get back, take your horse's pulse immediately, but not while he's eating/drinking/whinnying/being excited. You want an accurate pulse. Your target is 60. If he's already there, you're golden. If he's below somewhere 61-69, keep checking his pulse. As long as he's down to 60 within a couple of minutes, you're good. If he doesn't come down OR he's still very high (70-75 would be "iffy" while above 76 I'd be worried), then you need to go lighter. I highly doubt that's likely, given the work you've put into him and that any fit and healthy horse shouldn't have a problem coming down after only 4-5 miles of a working trot.
8) Adjust your training. If he's coming in at "golden", add distance OR speed - not both. If you're already up to 8-9mph good working trot, I'd just add distance, not speed. Any more speed really isn't necessary and strains your horse's ligaments which take years to build, not weeks. If you're coming in at "good", keep the same training schedule for a few weeks, then start adding distance. You'll get to know your horse. My horse always comes in at about 64bpm or lower, no matter if I went 5 miles or 25 miles.
9) Ultimately, you want to be conditioning at least 20 miles per week, but NO MORE than 30 miles per week. That does more damage than good. At least one of those days, you want to do hill work. If you can get in a short ride with tough hill work, that's great - but make sure it's shorter distance. You can count arena riding as conditioning, too! We were working on discipline at a working trot one day, and I got in all 8 miles. My friend does drill team with her GPS on, and she did even more than that! All riding counts as conditioning - even walking.
As far as nutrients goes, I'd say you definitely need to start with beet pulp, then add grain mixture. Soak the beet pulp to get a mash (you don't want any hard pieces) and it will help keep your horses hydrated. When you're just conditioning, this doesn't matter as much because your horse will have plenty of access to water most of the time. However, you want to get them used to eating it because it'll be a lifesaver at a ride, especially if your horse forgets to drink but is plenty eager to eat your beet pulp, getting his hydration that way. There are already plenty of threads on here about how to feed beet pulp, though, so I won't get into that many details. Just be sure to soak it, since hydration is the whole point of feeding it.
I add a Triple Crown Senior to my beet pulp because it's low in molasses, I'm already feeding beet pulp so I don't need a grain mix with that, it's high in fat, and has lots of other good stuff. I used to mix my own stuff together, but this is way easier and ultimately better and cheaper. You'll also want to get a good mineral powder to put in there, and electrolytes are great, but make sure your horse is used to them. I don't do electrolytes, but I want to explore it in the future. Ultimately, talk to your vet about what your horse needs. Since he's a different breed and build, he may require different supplements, but I don't think you can go wrong with that combination.
Read more: http://www.horseforum.com/endurance-riding/new-endurance-148510/#ixzz2J3AngBs9