Skyline Survival

With blue sky visibility and no snow this should have been a pleasant introduction to the legendary fell race the Edale Skyline. Veterans will know that there's really no such thing, but my oh my....! What a total nightmare I had today.
At 2 miles I was probably in about 50-60th place and consciously trying to slow down
At about 6 miles I was letting numerous speedy descenders (not my strongest suit) zoom past assuming I would catch them up on the ascents as I have done in shorter races.
At the 12 mile Mam Nick I was pleased with my 2 hour time and beginning to imagine a top 100 placing perhaps around 3.45hrs.
At 12.2 miles having stopped for a big drinks and gel refuel, I began to realise everything wasn't tip top; it felt like someone had left the cap off my fuel tank, and with almost 9 miles to go I suddenly felt dangerously close to the red.
At 15 miles I was well into survival mode, any ideas of a decent time had been dropped and just completing the thing would be a victory.
At 17 miles my right leg refused to work locking up in agonising cramps. At one point I wondered how long I would have to lie there before I was eventually airlifted to the human skip in the sky.
At 19 miles my whole upper body began to go numb with the only feeling one of pins and needles.
At 20 miles the cramps which had been appearing intermittently completely took over both legs.
At 20 1/2 miles I still thought I might not finish and was particularly worried about passing out in front of everyone on the final field.

Well I did finish in 4 hours 13 minutes and 146th place. I was pretty embarrassed by my performance - Ive had difficult times in the mountains before but I've never had the wheels come off so severely before. My guess is that there were two issues, firstly I think I got very dehydrated - despite drinking 3/4 litre it was surprisingly wind free and hot and I overdressed. The main one though was that I was under-prepared, the longest run I'd done was 1hr 50mins and that was on nice trails, what I needed was 3 or 4 3hr bog-missions. All in all it's been a very big learning process and with my second half marathon only 5 weeks away Ive got to knuckle down if I don't want to get punished.