Forgive me if I say something that seems obvious - having given lessons for a while now, you never know what people will assume about horses :P
- I've been raised with quarter horses. Yes, quarter horses have been the quietest "just do it" horses I've worked with - but I've also had one that was out of cutting stock that was HOT HOT HOT, as many lines are bred to be. As others have said, it all depends on the horse, not necessarily the breed. And while quarter horses are quiet, their spooks are in quality and many, once the rare spook comes, loose it and it's hard to get them over it. My arab, who I LOVE on the trial, spooks or shys occasionally (which is HUGE because he used to set back at the squeak of a gate!), he's so smart and works through it now. Basically, if I say it's ok, he knows it's ok. So it just all depends.
As for endurance ability, I believe endurance news just ran an article about some of the regular 100-miler draft horses, and I know a few ladies that ride drafts on our rides as well as a few ponies! If a draft can do it, any breed can. And any horse, conditioned sufficiently, can complete an LD. However, NOT every horse can be competitive, and not every horse can complete the 50+ endurance rides. Traits in these horses include an efficient mover (with more forward movement rather than an "upward" movement - this depends on the shoulder), good conformation of course (particularly in the legs), and a general willingness and happiness to just keep going. Trust me, you don't want a happy horse for the first loop, who thinks the vet check means they're done, and then you still have 15-25 miles of fighting your pissed off horse after he realizes the vet check was NOT the end of the ride.
Some quarter horses will, others won't. Again, see how efficiently they move, particularly at a trot. If you have to push, push, push to get them into a working trot (approx 8-9mph), it's not going to be a fun ride for either of you. I.e., don't get a western pleasure jogger :P On the other hand, one girl I know has to hold her quarter horse back because she trots so fast I have to lope behind her (thus why I started riding on my own - this was not good for my horse). Starting out, though, you don't want to be going that fast because you'll ruin your horse. Build slowly but surely ;)
Going into endurance with this mindset is dangerous. Even if your horse can be competitive right off the bat, it's not necessarily the best to let them do so. You can really do some damage. It takes time to build the muscle, lungs, and eventually the bone density required to prevent sprains and stressed ligaments, worst case scenario hurt the bones. This can only come with time. I've been told it takes 3 years of regular endurance to build a 100-miler horse. Until then, just admire the people who've worked hard to get there.
The best advice I got was to treat actual rides like a really long conditioning session. Eventually you'll "accidentally" top 10. There are two kinds of riders - those that compete for miles and those that compete for placings. Usually, these are two different riders and don't mesh well. The people I've worked with that are very competitive only ride the rides they think they're going to win (based on terrain, time, who's competing, etc), and pass up all the others. They've also been doing this for years and years - most around here since the 80's and even 70's. Unfortunately, we had quite the scare at the last ride when one seasoned rider won the 50 two days in a row. Minutes after the final vet check on the second race, the horse went severely downhilll and was in danger of dying for the remainder of the day. And when you're in the middle of nowhere for a ride, you do not want to need a vet. The vets there do not have all the resources they'd normally have, and treating a horse is their worst case scenario. The guy almost lost his horse - and the same guy DID lose a horse a couple of years ago at a ride. Please don't become this kind of rider.
Want to be competitive right from the start? Compete for turtle (last place) - it's quite a nice ride when you're not stressed about everything else, including overriding your horse, and you still get a prize! Turtle can get pretty competitive, at least around here - one time a few riders hid off-trail and waited for a friend of theirs (who ALWAYS got turtle) to pass them - it was quite a shock at the award ceremony when she thought she'd gotten it again until they announced it!
As the AERC motto goes, "To finish is to win."
They can do LD's at 4 (based on actual birthday, not Jan 1); They can do endurance rides (50+) at 5. If you plan on doing the higher milage (like 100's), I'm told you want to wait until they're 9 or so to fully mature. It's a whole new ballgame when you start talking about "training" and how young is too young - I'll leave that debate to the training forum. But for beginning an endurance horse, I think 6 or 7 is about ideal for starting conditioning, but definitely no younger than 4 for LD conditioning (I personally think that is too young though, especially for Arabs who mature later) or 5 for endurance since that's the limits you have to follow anyway. I've seen horses into their early 20's competing on endurance rides, and a few in mid 20's doing LD's, but it all depends on how hard you've ridden your horse. If you've ridden hard, your horse will not last as long in life. I personally want to go for the decade team - at least one 50+ mile endurance ride with the same horse and rider for ten years in a row. I won't make it there if I'm shooting to win all the time.
Anyways, that's my 2 cents. My biggest fear is people getting into endurance and just going about it all on their own and then ruining or injuring their horses. This is one of the most extreme equine sports and we ask so much of our horses - everything has to be done with the horse's health as top priority. Once it stops being the priority, you move closer to loosing a horse at the ride. Above all, find a good mentor that has goals similar to you and has ridden for years - as in at least a decade. You can find people who've competed for much longer. Good luck :)