My 2p for the OP.
Weigh yourself before you go out and again when you come back, the loss is mostly fluid and will give you an estimate of water loss and taking into account what you drank on the bike ( 1 Litre = 1kg) you can figure out your needs.
You should be able to do the first hour or so without anything to eat or drink, it's not a race. Then you need to consider what you need as others have pointed out. I'd start off with plain water and see how that goes. You are going to need some carbs to keep you going in some form or other and as gels are of little nutritional value I'd go for something very simple like jam and bread, banans etc.
Gels or drinks enter the body at roughly the same rate and take around 7mins from your mouth to your bloodstream but can create peaks and your body pumps out insulin to try and undo what you are doing if you use big doses.
You are looking to top up your system not replace all the energy expended because your body should supply some energy from your fat store. Most atheletes have around 75,000kCal of fat lying about and only about 3,000 kCal of glycogen stores. You want to allow your body to eat in to that fat store, not for weight loss but to improve your bodies efficieny and reduce insulin peaks/troughs. You are better off running a little low on the carbs to encourage use of the fat store.
Your bodies ability to rehydrate itself is age dependent. Your kidneys are 50% as efficient at 60 compared to a 20 year old so getting rid of fluid is harder as you get older and your body holds on to liquids longer.
Athletes do get short of salt too. One simple trick test whether you are deficient is to have a bag of salt and vinegar crisps when you feel you are loosing energy, having only consumed wated prior to this will give you some indication of their effect.
I'd suspect but cannot show that the salt deposits are probably from the salt laden cheese which is also what is giving you a thirst. If you are going to eat cheese on toast the unprocessed cheese and whole meal bred has a much slower burn rate than the processed kind. Its close to 50% difference. But as the other posters have said, porridge or my pref. muesli works better. (paper can be found here
http://sweatscience.com/digesting-a...ns-twice-as-many-calories-as-a-processed-one/)
One other section on this site shows that carb intake improves performance. (
http://sweatscience.com/higher-carb-intake-faster-ironman-finish/) which is great, but bear in mind that this is a race not a training session for which you should train carb depleted not carb loaded. This causes some confusion. For how this works search this forum for interleukin.
HTH
