Ditto all of the above, though perhaps I should listen to that advise myself.

Last year I set myself a tough high level route and put in plenty of miles of training in the months before. A 5 mile run a few evenings a week, a good 20+ mile walk over rough ground every weekend, and at least one night out each month.

Then with two weeks to go I went down with a chest infection which floored me, and left me completely exhausted and struggling to do anything physical.

So when I set out from Oban I simply adjusted my route day by day depending on how I felt, and got across without too much of a problem. And I think that's probably as, if not more, important than absolute fitness. Listen to how you feel each day and don't be a slave to your plans if you don't feel up to it. There's generally an easier route, or a short cut even if it means adjusting several days of your route.