GVH are hosting a 12 mile marshalled training run. Starting at the cricket club, Boxmoor. £5, registration form is online. Cake and a hot drink at the finish.
9am start- 12+ pace
9.30- Main start
I did that one a few years ago - very nice run (even if the tow path was under water!) and well-organised. The cake afterwards was excellent too. I'd recommend this even if you're not marathon training - good 'Beast' practice, maybe?