I'd echo what Phil said..... I've done the Midsummer Munro once before, and am doing it again this year, and the steps are a killer (both going up and down).
I regularly run around Box Hill, and My advice would be to get in the car and drive to some hills. However, if that is not an option, then do lots of walking up and down stairs, lots of gym work focusing on leg strength (especially calves and quads), and maybe do a couple of weekends hill walking....