Phil - I have not entered anything for that weekend - in past years I have started the taper by that point- but a lot of folk will do Halstead. There is also Stroud Trail Marathon that I like the look of.
The problem is that the road slopes one way across its full width, so it is similar to running along the side of a shallow slope (and swapping to the other side of the road would not work, as in the UK, as the slope is in the same direction). Over dinner a couple of days before the race this was something that Slotwin had mentioned that she had found a problem in a previous Down run and my experience was the same. I got a stiff calf and had to stop to stretch it out and it was a problem for about 20 miles. I never found it to be a problem on the Up run, for some reason.
For the last down run I did lots of one legged squats and plyometrics. You want quads of iron.
This time I will be doing more work on the stabilising muscles in the lower leg, tibialis posterior and anterior. On the last down run the cambers in Harrison Flats gave me a stiff calf pretty early on and I think it cost me a sub 9.
I shall also be recommencing Bikram yoga soon. It is good for getting used to exercising in heat. Namaste.