A bit of a crap answer I know but this is probably the truth of it. I've only relatively recently (since March I guess) started a proper running schedule and its worked wonders with me - most of my runs include a good hill or two and I tend to put in 30 to 40 miles pretty much every week (when I'd been doing 15 to 20 previously).
Jumping back to April though, I wasn't so fit and within a two week period I ran the 3 peaks solo (as a refresher
), did a fair bit of normal running, twisted my left knee and then decided to do the 3 peaks fell race on a pretty hot day and with a dodgy knee. I got round but I wasn't a pleasant sight at the end - I'd suffered massive cramp going up Whernside, massive cramp up and on top of Inglebrough but managed to stagger on to the end, running off most of the cramp going down.
But then I pretty much seized up after finishing with full body cramps - it took me almost 15 minutes to just get in my car.... seriously.
Since then though, although my running pace hasn't altered drastically, my cramp resistance and being able to finish a long fell run in goodish nick and a smile on my face have made massive improvements. I did the Wharfedale Off Road Marathon not so long ago and then Borrowdale this last weekend and felt fine after both, without a John Wayne walk and without crippling myself getting back in my car.