Since you're in Oregon you can check out this website: www.nwrides.com
If you look under the calendar section you can find all the rides for 2011. I have never done CTR but instead started with Limited Distance rides which are 25 miles. It is really fun as long as you love riding A LOT.
Endurance in a way is cheaper than showing but I have spent a lot on getting tack that fits well and on horse boots.
The main issues I've had are making sure things don't rub (a girth that doesn't bother a horse for a 10 mile ride might rub a sore after 25), hoof protection and getting my horses to drink on the trail.
Any breed is fine for a 25 miler. However if you want to win you are more likely to with an Arab. On my last ride with friends we had a Paint, an Anglo-Arab, a Connemara/Standardbred cross, and my Arab. Both my Arab and the Connemara cross will easily be top ten horses as we bring them along. At the end of 25 miles they are ready to do 25 more. The Paint was done mentally after 20 or so. It comes down to heart even more than the physical issues. Some horses really lose the will to keep going after awhile, even when they are passing the vet checks well. I have seen TWHs do really well.
The hoof issues can be tricky since on rocky trails in Oregon horses' soles can bruise even with shoes on. I prefer boots, but it's tricky finding which ones work for your horse.
My misconception when I first started was that endurance rides were walk/trot. They are actually mostly trotting and cantering, if you want to place. So imagine how much riding you have to do to get your horse fit enough to trot and canter for 50 miles.
Endurance people are really nice to be around. The rides have a great atmosphere.