In reply to Tommy Harris:
Sweating and salt stains is perfectly normal on a long run. I'd only get worried if you're hot and NOT sweating! If you're eating reasonably 'normal' food during the event, you shouldn't need any extra salt. The 'electrolytes' in hydration drinks are more to help the gut with water absorption. Also, remember the old idea that cramp was caused by lack of salt was debunked years ago.
The main think you have to learn with (UK) ultras is how to feed on the run - what works for you and what doesn't. If you are new to it all and slow (recommended) then pretty much any normal food is best. You'll quickly learn what your body craves (and it's sometimes quite weird and intermittent). The trick is to be disciplined about eating early, little and often - even when you're feeling a bit grim.
Also, remember: If you are feeding, drinking right and running at a sensible pace, you are still likely to have a 'bad spell' at some point. It is likely to be mental as much as physiological and will pass in time. Just have faith and plough on - you'll probably get your second (or third) wind in 5-10 miles time and be smiling again.
Post edited at 14:51