km, a lot of plans do a max of 22 miles. The last 4 or 6 miles will just happen during the race, I guess the adrenaline keeps you going - it certainly will if it's London! I have run them on 17 to 22 miles max training and I have to say 22 mile training was my best marathon, but I guess that was because I was generally fitter during that time.
I'm sure people will have told you this already but for the race and during long runs too I guess; start off slow!! If you want to run a certain time, break down into miles or km if you prefer, work out your pace per mile and go slower for the first half than the second. Never race off at the start 
Snow gone here, sunny at the moment.