Scafell. No "r".

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More than enough has been said about preparation : while you need some, your aerobic base will be great.
South East London makes me think of Pilgrims way and the South Downs. You can walk for miles and go up and down a decent amount.
I think just getting a bit used to being out walking for a few hours is a good idea, wearing your boots in, finding the right socks etc. and getting your feet used to the repetitive impact and blister possibility. Marathon runners do long runs slowly to get used to the way the body changes how it accesses energy, not to practice going fast as such, so its likely worth going on a few long slow walks of 4 hours or so.