Ran slow 10km in 1:06:12 to see if my legs could take the distance and a bit more and essentially not be entirely knackered the next day.
Hardest part was actually running slower than I wanted to. Usually I can cover 10km sub-50mins fairly comfortably, but I like to push myself and this is not a good idea when there's 3 times that distance to cover. Despite being dark I ran on the common ground, not only because it is nicer on my knees, but also there had been a lot of rain and there were tons of puddles and mud and so it forced me to slow things down a lot. Also the cars coming towards me on the road were a bit scary sometimes.
I'm fairly confident I could run at least 8 miles now at this kind of pace. I've also heard of people who run and then walk parts of the 26 miles distance and this is sounding more and more sensible to me. hmmmm.