Mouse - I'm no expert, in fact am a bit of an non-understanding numpty about all of this too, and am also just stepping up the milage for a marathon too (think we're doing the same one)
I've just starting experimenting with the sis go gels, whichever flavour you go for they're exceedingly sweet and the texture is, shall we say, interesting! but I have no idea how many, how often, is water ok or should I be drinking other stuff???
awaits answers from those who know...