After about 15 miles I start to feel pretty tired - so far I've just been carrying a bottle of water and a bar of chocolate - but all that does is give me indigestion - If I try & push my pace up a bit or it's pretty hilly then I seem to often end up with a blinding headache - so first question is - do other (older) folk find these electrolyte energy drinks help with that?
(I am younger than you, but past my physical peak).
Electrolyte drinks, or water with a little squash and pinch of salt, does hydrate better than plain water, and hydration helps to prevent headaches and other problems. Try a snack with less fat, eg. a good cereal bar or a piece cake. Ordinary cake isn't very high in fat, and remember that icing is just sugar, if you like icing.
My bike is a Giant CRS 3.0 hybrid with a gel saddle - seems fine for around an hour but then just seems to become uncomfortable - any observation on gel saddles would be welcome
Saddles are the most personal factor of cycling. Gel saddles work for some cyclists, but not for others, including me. As well as Brooks leather ones, try saddles with foam rather than gel padding. Foam can be dense and stiff enough to provide a reasonable amount of support. Avoid extremely soft saddles that you just sink into. Try as many different saddles as you have patience for, and experiment with the position of each one. Sell your unwanted saddles via adverts on this forum.
Regarding a custom made frame: there is only a big advantage in getting one if you have an unusual body shape, eg. particularly short back and long limbs, or vice versa. However, it is always worthwhile to have a custom selection of components (rather than a ready-assembled complete bike), including handlebars, handlebar stem, gears, pedals, as well as the saddle. Also get decent cycling shoes with fairly stiff soles, whether using toe clips or clipless pedals.
For the actual cycling, my tip is dead simple. Cycle when you like, where you like, for as far as you like, how ever you like - except for most of the while, cycle SLIGHTLY harder than is very easy and comfortable. Only slightly. This will gradually improve your strength, fitness and endurance. Don't be ashamed to avoid bad weather if you don't enjoy rain and gloom. Cycling is supposed to be pleasurable (when you're not a professional racer).
Wishing you a pleasurable 2012, Ian.