hello TT76 have you tried taking Echinacea, it boosts your immune system, it was recommended to me when upping my mileage training for London marathon this year, it seemed to help and am using it in my training for Snowdonia mara also, and eat well.
The only advice I can give you for your training is listen to your body, if it needs a rest give it a rest, and long slow runs are exactly that don't push the pace 1 - 1.5 minutes slower than your predicted marathon pace. As Insomniac says, Snowdonia is not as bad as the image the name portrays, it is Snowdonia not Snowdon, except for mile 21ish, big climb.
There is someone on RW who has an online coaching forum Micksta I think, have a look for it, if I find it I will post you a link.
Happy running 