I can't really help you much because I have two friends who are former record holders for the Tube Challenge; I myself jointly held the DLR record (with one of the two) for a brief period in September.
What I will say though is this...
The tube Challenge site is your friend here, but as others have said they won't give away much in terms of planning. NR etc are permitted but the goal is to visit every station on LUL, so as far as I know you have to call at each station on an LU service (so Chiltern at Amersham wouldn't count). Using the DLR and LOROL is of course allowed but again they count separately.
Try to find an ideal start and finish point (two from Upminster, Epping, Heathrow and Amersham/Chesham are the usual choices) and plan to join up separate routes by bus or on foot (e.g. Cockfosters to High Barnet).
As I said, I can't really say anything else, to protect my friends' interests (one of them wants me to do the Challenge with him later this year), but realistically anything above 18h isn't going to be a threat.
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Regarding charities, find some that mean something to you. That is the best advice I can give.