I don't think it's in the same league as some of the ones mentioned, but if you want 26.2 marathon distance Neolithic Marathon might just sneak in somewhere... Also if you wanted to have a spread of different areas of the country?
On measures of hardness, I don't think it's just about hills. Hard and rough ground, mud/dust, rutted tracks and stiles definitely add to Neolithic's hardness quota.
(Should say I've only done FLM and Neo, so can't really compare with the others personally. Neo is a lot harder than London, and includes some steep hills, but not fells and mountains... it's also a great event, with walkers and cani cross events along the same route, great organisation, and there's something special about doing a course from Avebury to Stonehenge. If you were looking for 7 unique marathons I'd definitely nominate it!)
Edited: 02/06/2008 at 18:00