In reply to OG:
Wow this has been interesting.
I've gone from road half marathons through to a small number of 40-50km events in the Peak, plus overnighty and Munro-baggingy outings. What I have learned, some of it uncomfortably is...
Uphill: beyond a certain angle, lots of fast people walk. Some people run. If you're competitive and you care about biomechanics, you might do better running, but otherwise a walk can be fast and effective.
Nutrition: you gotta. A physio who had reason to know once told me "at those distances it's basically a picnic with some jogging." Find something you can eat and digest on the go. For me it's pepperami and oatcakes, or peanuts and cheese, plus a few flapjacky things and or jelly babies. Partly because they in small easily-unpackable quantities which don't litter. (Footnote from running into rainy wind - I have learned not to swallow so much air while trying to chew oatcakes and breathe rain/snot/crumbs, because the cramps later are not fun.)
Salt/rehydration: water obviously. Less obviously, I was having some leg cramps which might or might not have been about salt replacement (probably not at my tiny distances ?) and was trying tailwind and focusing on getting food in. I didn't buy any more tailwind (nothing against it) and I'm none the worse for it so I suspect that for me it's more about....
Cramps: tight calves, initially gastrocs then later soleus (lower down the leg anyway) - significantly looser shoes and rolling with a PVC pipe have sorted me out longer-term. Occasionally tight hips. That's obviously personal/age/lifestyle stuff. If you've got any musculo-skeletal tweaky bits, or you suspect you might, you might want to pay attention to how they react to uphill. I blew my calves up on the Nine Edges a couple of years ago, hobbled a lot of it, and thus have the opportunity to try to take a good chunk out of my time trying again this year...
Downhill: what they all said made sense. I like running downhill (mwah hah hah) so it didn't matter much to me... equally my comments above may not apply to you !
Y