Hi KarenK - in terms of trashing my legs I think road marathons always got me more than ultras. Guess this is down to change of terrain, gradient and also that people tend to walk the uphills at a decent pace as it is more efficient than trying to run them with an increasingly shorter stride (obviously if your name is Lizzie Hawker or Jez Bragg this walking business doesn't apply to you!).
You will be fine if you can do road marathons. Just go off easy and bet you'll think pace is easy compared to pegging round a road marathon.
My one bit of advice is, that if you can, reccie the route, especially the leg from Ambleside to Coniston. If you can only do one reccie that is the bit to do as for most mortals a chunk of it is in darkness and there are one or two tricky bits of nav (well tricky when it's claggy/dark/your shattered/all three).
Looking forward to the 50, it's awesome. Hoping to get out and reccie next year as didn't do this year's race so my memory of route a bit hazy.