It was my first marathon , so its definitely doable - assuming you do at least some off road running though. First.half is hilly but very scenic, second half was a grind through muddy fields for me, very churned up and loads.of gates and stiles. However, I think that in someways this made it less punishing over all
It is informal and friendly, and generally well signposted. You can get lost though...I did! There's a map, but it is extremely detailed, and not really usable for runners. Also the terrain means it won't be fast.and the majority of people are walkers. If you don't start too early, they make useful signposts at least to halfway!
If you are building up to a marathon, have some off road experience,want a friendly less competitive environment, I'd say go for it....I recommend tea and Victoria sponge from the WI at the end.