London Marathon - thinking of entering

I'm thinking of entering for the London marathon in April.

I know there are many people here experienced in combining distance
running and a lowcarb WOE, especially by cycling carbs.

Is there a book or URL that gives helpful info on this?

FWIW, I'm now up to 8 miles on weekends, 6 miles every day (I don't get
stiff or sore at this distance). My pace is quite slow, though - 5.5.
miles per hour - but then my legs are really short, and this pace takes
my heart rate over 150.

My current carb intake is around 25-30 carbs per day. I guess what I'd
like to know is how far do I have to be running before I need to carb up
ahead of time? I've never needed to so far, though some days it's a
slog.
--
Jane Lumley
179/141/140