BANDERA BANDERA I love that place --- it is 4 hours from me - I have friends there that practically have the whole place as their backyard no fair...
The trail maps are fairly accurate (if you were using theirs) since I know those ladies/gents actually ride their trails with a GPS. That is a great place to train because of all the different types of terrain and switchbacks...try trail 4a/b sometime - go up, but coming back DOWN will scare you to death!
I usually start around with trying to ride three times a week, and ride at least 5-6 miles each time (that equals 5/6 mph). I use the Garmin 300 (comes with heartmonitor) and bought the kit that turns it into a horse hearmonitor, and then I can watch his heart rate, see my mph and distance on my wrist. Piece of cake.
I try to keep his heart rate around 135ish for the first couple of weeks, until it seems like he go farther without it going up, then I just double my time on the trail, but not the speed and watch the heartrate again. After a month or more of this, then I increase the speed a bit (mostly fast trotting) for those 3x week rides, with the overall mileage being around 20-25 miles a week.
Then I just do that over and over for another few weeks or so then I add a 20 mile ride on the weekend here and there (trying to finish in 4 hours or so). By now, at least 4 months have passed and as long as he can hold a nice 6 mph with his heart rate holding at 135 while he's doing it, you can do a 25 LD no problem.
But then I'll start another horse the next season and he may/may not work out with this program - he may be very unfit and it takes us 6 months to train, or he may be so fit that we up our trotting to 8 mph and follow the same program.
Either way, don't even plan on going super fast the first season - plan on the entire first year for building tendons and ligaments because that's how long it takes, even tho your horse can be cardio fit in 3 months. You'll end up with injuries that will sideline you for months if you don't heed my words!
Keep riding thru the winter, slowly increasing speed and mileage and then you can go for the top 10 the next spring in LD...
My plan is very plain - others have much more elaborate training tips they can offer - mine is what it is because I have to work around kids/family/farm etc.
Do you have heartrate monitor for your horse? It helps to have one in the state natural area - I have backtracked with my GPS many a time - it saved my butt! LOL! Plus - if you like Geocaching there are quite a few geochaches hidden there that are only accessible by horseback - it will give you a goal so you don't get tired/bored too quickly.
People hide stuff and leave coordinates to find it - it is usually a bucket with trinkets of stuff to trade - LOADS of fun!