The advice is based on the warm weather which is forecast for Milton Keynes with temperatures ranging from 13C to 17C with bright / sunny conditions. Sounds great but there is a risk we runners could over heat as we are definitely not used to these conditions (a month ago there was snow on the ground). Even sunblock could be a considerations for some.
Here's their helpful link through to the Runner's Medical Resource - http://www.runnersmedicalresource.com/drinking.html
Mind you, I doubt if I am the only runner who remembers the horrible conditions last year; these were quite the opposite! Why not take a look at my account of last year's marathon - Click here - and for those not there, this photo might give you the idea
Here's sending best wishes to all other runners taking part on Monday - good luck, stay safe and enjoy all 26.2 miles!