I did a twenty five mile a few years ago. I rode 5 days a week for about 2 hours a day. If I wasn't able to ride, I lunged for about an hour (30 minutes each direction). Obviously, make sure you start out slow and work your way up. We started out just walking with a bit of jogging. Towards the end we were trotting for the majority of the time with walk breaks and short sprints.
I like to train on different terrain. Hills, sand, packed dirt, etc. Also, we had to go through fairly deep water (about hock high) on our run so I would make sure your horse is okay with water.
Make sure he is getting the nutrition he needs and gets proper warming up and cooling down. It needs to be done at a pretty forward trot. Ours took about 4 hours (with a check point) give or take.
Do you have a riding buddy? Most of them send you out in groups of two. My group was me and the girl I had trained with. It made it easier because our horses knew each other and it was just more fun :)
Gow it worked was we got a vet check. They checked for any sores, windpuffs, splints, etc. They marked this down (I am looking at my score sheet...I will try and see if I can scan it). Then, in hand you trot your horse in a large circle. They look for length and hight of stride, body carriage, willingness, toe dragging, coordination any soreness, and muscle tone. Then they look for your horsess pulse, resp, and temp.
We had one vet check in the middle where they checked the vital signs again. Our horses got to rest for a few minutes and get some more to drink. Then we finished and everything was checked over again. They deduct points for anything. For example, I got a half a point taken off because his stride was slightly shorter and another half taken off because he was less animated. So one point taken from the Mechanical fatigue category.
Then I got another point taken off because he was kind of tired. So my body condition score went down from 100 to 98. You can get peneltys for behavior, thumps, or time. (Youre supposed to complete it in a certain period) We got 4 taken off (We got lost haha). So my final score was a 94 out of 100.
For referance, his pulse during the vet check was 36 and his resp was 12 during the vet check. When they checked at the end, it was 40 and 6. I got zero marked off.
Vet checks vary from 25 (done in about 4 hours) to days long. 25 can be called an extended ride/long distance ride and it's a lot of fun and not horribly strenuous.