This is great info! I am currently trying to get my Haflinger to do LD's, but am worried about the conditioning! Just going to take our time and see where we can get!
Yes! You have 6 hours to complete the LD, which, in reality, is a lot of time for a reasonably conditioned horse. We had our buckskin QH complete, and he's the laziest horse ever - he would go backwards if he could. It really IS important to do long, working walks. If your horse doesn't have any glaring conformation issues, his back should be fine if your saddle fits well and you are not a completely unbalanced rider - if he does get a sore back, check the saddle fit unless you know he has a long back or back issues, and then you should consider another mount for endurance. The long walks teach metal fitness as well as physical strength, which is really important to prevent the horse's mind from 'turning off' (also, these are 'working walks', not riding on the buckle spacing out - ask your horse to pay attention). While it is more fun to trot shorter distances, the walk teaches real endurance, the mental kind. Definitely read Endurance 101 and take your time when conditioning. Ideally, it is good to build up the mental fortitude over a month and then go into trotting, especially if your horse and YOU aren't used to long hours (remember, YOU will be in the saddle for hours and hours, too, and it takes patience, and maybe some Advil). The walking will build a base of strength and a good mind, and the trotting/cantering will build on that. There is a nice, concise PDF put out by SERA that talks about conditioning: http://www.seraonline.org/Conditioning.pdf
It has some basic information and good suggestions for workouts if you are wondering what to do.