The beauty of a long fell race is that even if you do start too quick you can ease off as and when the course allows ie. walk the steep bits, run the downhills easy etc.
Running marathons on the road, in my opinion, can be much tougher because once you have committed yourself to a pace, by the time you realise you've started too quick it's usually too late to do anything about it. All you can do is try and somehow survive to the end.
Also fell running is far less draining mentally as there is always the challenge of the next top, the next CP etc. rather than just miles and miles of never-ending tarmac.
I know which I'd rather be doing...
MH