Ideally find someone you trust - who also spends the time to do the work on the latest and greatest theories in nutrition and sports science.
I am a fan of Noakes, read his book, listen to his interviews. Drank to thirst - got extremely dehydrated and had a REALLY bad time on race day (admittedly disgustingly hot european ironman but still I was annoyed). So, stuff drinking to thirst. However I have never had a problem drinking to thirst on a shorter race like a marathon.
I know Noakes has gone paleo so he will be banging on the low carb/high fat drum. Personally I have been there - tried it (after all, why not) realised I can get away with fewer carbs than I thought - however.... realised I need carbs not fat to do super long distance and anything at all that has any oomph.
You will have to make up your own mind but the high fat low carb thing is hard for an endurance athlete - even the ones that do it (from what I can see from my own internet trawls) still stack up on the carbs (sweet pots and stuff like that) before and after training sessions and STILL use gels. I even heard an interview with the 'super fat burning man' (that's the name of his podcast can't remember his real name) - he is one of the gurus of fat adaptation and eating fat and protein on long long runs - and even he said that you need to go slower if you cut out the carbs.
So,in summary. Fluid - try out on long training runs and find out what suits you - just don't guzzle gallons and gallons and think that the tiny amount of sodium added to sports drinks will be enough to offset too much liquid. Remember that sports drink companies live to sell - they do research to prove their stuff is the biz so don't believe anything they say unless you can find unbiased info that backs it up. Use sports drinks for their carb content rather than their hydration properties.
Gels/jelly babies etc - don't deprive yourself if you want them - they are a great tool. Who in the real world actually exists on these extreme diets anyway - life is too short to 'fat adapt' (IMHO).
What a great thread - now thinking back on all the painful things that have happened to me - post surgery drains (2 off) being removed and one getting stuck.... never felt anything like it. Although trying to sit up 4 hours after a c-section was probably worse but I have blanked that one out for fear of PTSD.