I didn't read all the posts because there's a lot of info just about endurance there and I just want to deal with the original breed question, plus the bit that Joe4D said about walking. Let me tell you about the Fjord that was at the training ride I went to last year:
This little guy was heavy, swaybacked, and his feet were overdue for a trim. I saw that a young teen or maybe tween was going to be riding him, so I stayed mum. I really, REALLY wanted to question that this horse should be going on the ride at all. But he wasn't lame, he was alert and responsive, passed the vet check so...
Talk about put me to shame on my Standardbred :) That little guy walked so fast I had to trot my 15h horse every couple dozen yards to stay with him. He WALKED at least 6 mph, no kidding. He could walk the entire trail and come in well in the time alloted without any exertion whatsoever! Plus, my girl ended up lame because she wasn't used to the type of rock on the trail, so we bowed out.
So, yes a Fjord can do it and yes some horses can do it at a walk the whole way and not be exerted at all.