The site wasn't very specific about how far/fast to work your horse in conditioning, just to do it about 3 times a week, so how far do you generally ride for a conditioning session? That way I can know what to work up to without overdoing it. Thanks again!
One of the big things you will hear experienced people talk about is starting a new horse with "LSD" (which is "long, slow distance"). As someone already said, the cardiovascular system gets into shape pretty quickly, but tendons, ligaments and bones take longer. They are what need the long, slow distances to make them strong over time.
Working 3 times a week can mean different things to different horses, depending on where you are starting from. If you are taking a horse who has just been hanging out in a pasture for years, you are starting at a very different fitness level than one which has been used to 4-5 days of ring work (even "easy" ring work).
When I first started conditioning Dreams, she was borderline obese, having been standing in a pasture for almost a year without work. We started conditioning in late November with the goal of being ready for a competition in April.
We started dong short, slow rides (covering 6-8 miles in 2ish hours) twice a week (which was the most I could ride in a week due to travel issues). After maybe a month, I bumped that up to one 6-8 mile ride, one 10-12 mile ride, keeping the same moderate pace (though I would train at all 3 gaits.. just did a lot more walk and slooooooow trot than anything).
After another month, we bumped up to 2 10-12 mile rides each week. Then we bumped up the pace on one of the rides, but kept the distance the same. After a few weeks of that, I went to a faster ride of 10-12 miles and a slow ride of 15-18 miles. Beyond that, I slowly crept up the pace of what was considered "slow" (for me, an overall pace of 5-6mph) and "fast" (overall pace of 8-10 mph).
Once we started competing, my conditioning backed off a lot. My first season, I was riding in a 50 miler every month. Dreams would get the week before a race and the week after a race totally off. The 2 weeks in between, I would get her out once a week, for a moderately paced ride of 12-15 miles. That was more than enough to keep her fit (she also lives at pasture 24/7, so was moving around on her own all the time too), and as she has a good brain, I didn't have to worry about her losing her mind with that small amount of work. [One of my good friends has a mare who needs to be ridden 3 times a week or she will lose her marbles.. so that friend just does short rides of 4-5 miles each time, cause its the mental exercise she needs, not the physical.]
After that first season, she got about 6 weeks off totally, then we were conditioning once a week. Throughout the winter, the weather had a lot to do with how far and fast we went, with the goal being 15ish moderately paced miles. Once we got 2 months away from the first competition of the season, I started alternating between faster rides of 12-15 miles and slower rides of 18-20 miles (but still once weekly). Just how much condition a fit horse holds onto is amazing!
Our second season, we did our first 100. My conditioning program didn't change, as all the experienced riders I knew told me that the best way to condition for a 100 was doing a 50 miler. So we did 50 milers in March and April, then I did my own 2-day 50 mile ride at home in May (no 'real' rides local to me that month), then rode a 100 in June. Dreams got a month off after the 100, then I went back to the same schedule of conditioning I had been doing before to finish out my season with a few more 50s.
I think you have the right idea of getting to know your horse in the arena first and then moving out onto the trail as you both learn to trust each other. Any remotely fit horse who is ridden reasonably can do a 25 mile ride. You may not win, but you will finish, and that should be your goal to start with! Actually racing
in the rides will come with time.