I had a great ascent half of the marathon thanks to all the advice here. Got to the top within 2 minutes of my goal feeling very fresh and ready for the descent. Then, my vision went double (which has happened to me before with eye strain driving home after trail races, but never DURING one!) and obviously, it's hard to descend the trail very quickly seeing two of everything and having no depth perception, so I had to slow shuffle almost the entire 13 final miles. I was stumbling like a drunk because I couldn't place my feet, and I know you're thinking what the aid station people were thinking, "Is this guy all right?" I was, hydrated, cool, legs had plenty left: I just couldn't see well enough to place my feet. I stopped at Barr Camp and drank two entire cans of Coke because when this has happened to me out driving, that's fixed it before. Not on Sunday, alas. (Out of an abundance of caution, I even had medical check me out after the finish, where they gave me the thumbs up, and went to attend to the many folks who were seriously delirious with exhaustion.)
It was a great experience anyway. If I sort things out with my ophthalmologist, I will have some unfinished business to attend to in another year.