Choella, I also saw the Brownlees a few months back. They were on their bikes and were stopping for a "comfort break" as I passed!
Re the original question, I find walking 26 miles is easier than running 26 miles.
Each year, I do a 40 mile walk over The Ridgeway in Oxfordshire. The day finishes with me often feeling all in, but it never gets as hard as the final miles of running a marathon.
Those unfit enough to be struggling in walking, looking wrecked at 7 hours would probably have been hospitalised if they had run the whole distance.
Jelleybaby - try;
Visit the official Runner's World page
Follow Runner's World on Twitter
Other Natmag-Rodale Sites
Run For Charity
About Runner's World