Don't panic. HIgdon's fine ...I used it for a number of my marathons. Unless you're shooting for an aggressive goal and need specialized training, as
@gruecd did with his sub-3:00 race, most any of the plans will suffice. At heart, they're all quite similar - put in the miles, get in some long runs, work in some hills or intervals. You have a particular advantage with the leg strength that comes from the mountain work. That will mean a lot.
Doing a bit of walking during the race is "allowed." If nothing else, it helps to control the HR. You could just walk through the aid stations to ensure you're getting in good fluids (instead of drinking on the run). The fluids will be particularly important if you're at warmer temps than you're used to ...I see the heat acclimation as potentially the biggest issue you face.
eta: As I recall, Higdon mentions the consideration of finishing long runs at a faster pace, which is what the above link talks about (and fits grue's increasing-pace training runs). So the simple adjustment is to pick up the pace at the end of the long runs.