If it helps, I did Rombalds Stride in 3hr 51 the other week (approx 22mi/3000'), and although I'm not too bothered about what time I do for 3P, I've got half an eye on 4:30. If I was doing a road marathon, I'd be looking at running 3:15ish (maybe a bit quicker - my half time (1:25:40) predicts 3:02, but I'm sure that's overly optimistic.

I am in no doubt that I will be one of those lying down at the top of Whernside (or Ingleborough) crying in agony with cramp!