In terms of pacing strategy, your best option will probably be to walk all the uphill sections from the opening gate, while running the flat and downhill sections. When the organisers publish a course profile, I will be able to recommend a more detailed strategy.
You have until October to prepare, so your marathon training plan is a reasonable starting point. The main change would be to introduce back to back long runs. For example while your long run might have been 18-20 miles for the marathon, you would do something like 20-25 miles on the Saturday, then do 15 on the Sunday while your legs were still tired.
The South Downs way is considerably better than anything I had to work with in Leicestershire. You can always find some sort of section of steep or technical hill, and just keep blasting up and down it.
I don't know exactly where you live, but if you are quite close to the North Downs Way, the the ascents are a bit sharper and more technical than the SDW. A lot of CCC and UTMB entrants who live in the south, seem to use Box Hill.
For a project of this importance, it might even be worth your while to take a trip to the Peak District or Snowdonia, to get in some high quality hill training sessions.