Conditioning is what you do after your horse is legged up and you have a few miles on them. But the same principle follows almost every thing you do with your endurance candidate. A "slice of work at a time", never increase speed or distance at he same time. That goes for conditioning or legging up. You just follow the process all the way to the "fit horse". You will ALWAYS race slower then you train..... I think lots of people make that mistake, racing faster then they train, hence injuries and metabolic issues. The horse that wins the ride, and looks they just jumped out of the trailer is how you want your horse to look.
You are not going to go out and train for 50 miles, so you need to have shorter harder works that will help the horse get in condition for that first 50 miles. After you start doing some 50 mile rides then you can throw that into your training mix.
In order to get a "fit horse" , at least what my view of what a fit horse it, you have to stress that horse. So, what that means is that if you ride every day for 10 miles, just the same everyday, you have not accomplished much in the way of increased fitness. Now you can do 10 miles and you can do 10 miles.... You can break it up into mini sprints, pick up the trot, canter some etc etc.., so there are too many ways to count that you can do that 10 miles. So, what you have to determine is when you are stressing your horse a little. I don't believe in riding to failure, like in the human conditioning, but you do need to find the happy medium.
So how do you know when your horse has been stressed by the work you are doing. One of the best ways is by recovery heart rates besides "just knowing your horse". A new horse will probably not have great recoveries, depending on the work out but will grow into them. I put recoveries into two categories, working recovery, and resting recovery. On working recoveries I am looking at how fast a horses heart rate recoveries to 120, that is what I have used for many years. Of the two I put this at the top of my list. If you run, walk, canter, trot your horse up a hill and their HR gets above 170 -210 for a minute or two and they do not recover to 120 at the walk within 90 seconds I would probably slow down some. Of coarse I am assuming the you have reached the top of the hill and are now walking.
I used to be afraid to work a horse very much if their HR is much over 160, MONK proved me wrong on that. The key is that when slowing, they must recover to 120 or less within 90 seconds.
Less is More
After your horse is legged and pretty fit and you are in the conditioning phase, lots of trail miles can work against you. If you are not stressing your horse you are not gaining any fitness. Because you want to train faster then you race you don't want to do many "JUNK" miles. There is a place for "JUNK" miles but it will not make your horse more fit. When I say lots of trail miles what I mean is that if you go out to do a 20 mile conditioning ride and 10 miles of it is walking, you are not doing much that 10 miles. Now you could break that 20 miles up into the walking portion being your recovery. Sometimes it is hard to find the idle place to get good quality work without the "JUNK" miles, so you have to go with what you got. The one big training hill that I have, I have to trot 4 miles downhill in order to do the 6 miles of up hill, but it is a continuous uphill and very hard so it is worth it to me. I then have to ride pretty much level and downhill back to the trailer which makes my 6 mile training hill about a 16 mile ride.
If you are going to race downhill you pretty much better get your horse used to going downhill. I am not a big believer in fast downhills, but I condition my horse with my riding weight of over 200#'s and he ends up racing about 40 to 50 pounds lighter so that helps some. As far as downhills go, if you are going to go fast in a race downhill, condition your horse for that, don't go any faster in a race downhill then you would in training.
Uphill training is lots difference then downhill, you definitely want to train much faster on uphills then you ever would for racing. You only have so much gas in the tank, and you can't afford to run out or waste most of it racing up some big hill, especially when you have 20 more hills to go. This is where you can really excel with your horse if you condition properly for uphill. I live in the hills, so uphill is something that we do everyday, almost any thing we do we will have at least 500 feet of elevation gain. Sprinting hills is something that you might do in training but not in racing. You might have a hill that you sprint maybe 10 times in a row once or twice a week. Even walking steep hills on your easy days can be very beneficial, a way to turn "JUNK" miles
Rest, Supercompensation, Recovery nutrition, Barefoot, Boots, Tack, and more........