Never think about the whole thing! if you know the route, just think of it in sections with landmarks and just run each section at a time. if you dont know the route, eg in an unfamiliar race, do 5k at a time.
I agree with the others, 2 litres is an awful lot to drink.
Also, i would think that adding electrolyte tabs to sports drink is not the best of ideas,
the tablets are designed to be added to water to make an isotonic drink, ie one that is of the right concentration for your body to absorb quickly. if you change the concentration by adding to a sugary sports drink, presumably they are not going to work as well. (hence the cramps?)
what is the reason for drinking the sports drink? for energy or for electrolytes?
if for energy bear in mind i think you only need about 20 - 35g of carbs an hour ish thats about a third to half of a bottle of lucozade so you are drinking enough for 8 -12 hours! i would suggest using just the electrolyte tabs in water only for hydration (sip little and often) and then adding carbs in another form (eg gels, jelly babies etc)
in addition, remember that running slightly deydrated is not going to harm you as long as you hydrate afterwards properly.
Think our train is half past 1 ish on the Friday so not the same! I'll look out for you all on the day, but i am sure there will be little chance of spotting anyone given the amount of runners and especially because you'll all be way ahead of me!