I've been trying to compose a coherent reply to all the replies, but I can't, so here's a collection of thoughts. It makes sense to me.
I don't mix with other mountain bikers much because I prefer to
go somewhere, not just ride round and round in circles at a trail centre.
I think of myself as an Off Road Roadie, or as someone else put it, I go Two Wheeled Rambling.
I live in the Wyre Forest, virtually an unofficial trail centre, and while other mountain bikers are loading their bikes on to their cars and driving to the forest, I'm riding my bike away from it in to the surrounding countryside.
There will never be a simple yes/no answer to the cycling on footpaths question. There are footpaths round here that I have been commuting on for years without meeting any walkers and there are bridleways on the Malverns that I wouldn't even try to ride on a summer weekend. Don't be a dick sums it up nicely.
I don't like the idea of out and back rides, I'd much rather do a loop. Even if I've got a destination, I prefer to return by a different route.
I've never used mountain bothies, but I understand they work on a system of responsibility and discretion.
Reading a thread about HONC on STW once, some of the locals mentioned that the official route didn't use the best trails, presumably because the organisers didn't want them wrecked by 1200 riders.
I know a lot of local footpaths that make good mountain bike trails. I'd be happy to share them with a few responsible users, mountain bothy style, but wouldn't want a 1200 rider event routed along them. I would guess that most riders could say the same for their local area.
I organise mountain bike orienteering events. If you enter one of my events around the Wyre Forest or Clee Hill, you can be sure that all the control points will be on good rideable bridleways and all the unrideable stuff will be marked on the map as out of bounds. I use my local knowledge and experience of getting stuck in bogs or tangled in brambles so visitors can stay on the best trails for a grand day out.
I prefer to ride from home. Apart from racing and orienteering events, I can't remember the last time I drove somewhere to ride my bike.
I've been looking at maps a lot recently, and to get to the proper adventurous areas from here, Clee Hill, Wenlock Edge, Long Mynd and beyond in to Wales, is impossible without a lot of road riding, even if it is mostly single track lanes.
There are lots of mountain bike route guides about, but they have to be 100% legal if they are published for the general public. If they were a bit more discreet, they could include the sensible cheeky stuff.
So, to summarise, maybe I'm looking at two different things;
Proper ITTs are best being fully legal and following an established marked trail.
They can start out as an overnighter, but sure enough, someone will soon go for the record.
There's room for a lot more unofficial off road routes in the UK using local riders knowledge to link up the best trails with the most tolerable roads.
I really don't know how best to collect or share this knowledge though.
Worcestershire's fastest veteran vegan mountain bike endurance racer with a beard.