Training for endurance

[April Lee <adlee@bigfoot.com>]

Hi,

  I want to start training my horse (and me) to do endurance riding.
Apache is 15 years old (I think...) and is an Appaloosa.  I have owned him
for 4 months.   During that time, I have ridden him at least twice a week
for 1-2 hours at a time.  Before I bought him, he was in a small pasture by
himself for 8 months and was rarely ridden.  He wasn't skinny when I bought
him, but I have put about 100 lbs on him and he looks pretty good.  He had
almost no butt muscles when I bought him, but trotting and cantering up our
local (small) mountains has really helped.  I also am working with him on
backing and I heard that builds butt muscles.  Anyway, I took him into our
makeshift arena for the first month I owned him to get him more in shape
(both in body and in attitude) before going to the "outside world."
Saturday, I rode Apache for 10 miles.  5 miles one way, and 5 miles back
home.  I hosed him off and let him rest for 45 min in the middle of the ride.  

I'm curious about what you guys think about how much it would take him to
get into a position to do a 25 mile ride.  I'm not asking absolutes...I
know you guys don't know my horse...but I'm asking for opinions and
guidelines.  I don't want to kill or maim my horse...just wanna have a
little fun.  I'm 23, healthy, and have been riding for years.  I'm more
worried about his health and fitness than mine.  Besides, as I train him,
it'll train me, right?  :)

Thanks for the input
April