I second almost everything that CC said above.
Distance, speed, and the time it takes to develop each are separate, so I'm going to address them separately.
It takes 3 years to develop a 100-mile horse. The lungs, muscling, etc. can condition quickly and are only a matter of working your horse out and getting them used to the distance. For the shorter distances, such as LD's and even 50's, that's all you need and so it takes only a few months to get them ready. Many riders I know condition their horses and successfully ride them in 25's and 50's quite a bit their first year, but don't over-enter them in rides and give them adequate time off as well as ride them at a medium pace that the horse is comfortable with and not pushing for placing. For instance, I did do one multi-day ride where I rode my horse in a 25, had a day off, and then did a 50. Then, the second year, they start doing rides more often and start doing back-to-back rides, still taking it slow (but not TOO slow, as that can sometimes lead to more issues as well!). Finally, in the third year, they start adding more distance, like 75's and 80's, and maybe even testing their luck in an elevator ride for a 100, but I probably wouldn't do a 1-day 100 until the fourth year. This would be a rough plan for a rider who really wants to progress as safely, but most riders I know take their time knowing they and their horses are in no rush.
Now, for speed. Here's the best quote I've ever heard about adding speed, so I'm just going to copy and paste it. I've posted it in another forum, but it's just so good and lends a lot of perspective to how and why you should add speed:
I believe in the "2 years or 1,000 miles of competition before going fast" theory. I stuck to that with Shayne and again on the horse I'm bringing along now and it seems to work well for my horses and me. When I started endurance riding I heard that phrase (2 years or 1000 miles) allot and many people were pretty religious about it when bringing up a new horse. It has really stuck with me even though I don't hear it much anymore. I need/want/like my horses to last a long time and I believe that this is one way of stacking the cards in my favor to make that happen. I don't creep along the trail by any means, and I do spend a great deal of time conditioning my horses for the rides. But, I try to be conservative/middle of the pack"ish", for the first 2 years or approximately 1000 miles. I have also learned to be focused but flexible; that competition is not only about coming in first, and that nothing lasts forever.
Don't let your competitiveness carry you away. Put the horse's well being before your own. If you think your horse is off...he is. And, realize that at this time next year (or possibly even next month) no one is likely to remember, or care, where you placed at this ride or any other ride. So, take advantage of opportunities as they come but don't spend your time worrying over placings at a ride or push too hard to try to make something happen. The things that people will remember are how you treat others and how you treat your horse. Suzanne Pindar