Mark - yes, I expect you will get under 6.30- in fact I would recommend you aim for 5.59.
my DD and marathon times are similar to T-Rex; average marathon about 3.38, fastest DD 5.18. I walk up the hills - the first one being at 10k and the second at 20k. Until 20k I these are the only ones I walk, but 20k - 30k is quite hilly, and there I do a fair bit of walking. After 26 miles there a nice 5 miles which is quite flat and most runners speed up.
My corpse reviver has three tablespoons of glucose to one teaspoon of salt, per litre, although it needs to be stirred quite a bit to get all dissolved.
I dont think it HAS to be lemonade Corinna, just as longer as its got lots of sugar. Coke may be a good alternative as it has caffeine too, but I haven't tried it.