Well Col, not knowing what part of the country you live in, hard to suggest too much, if you live down south near to the SDW, I would say go and ride some of it , road miles will help, but you cannot beat getting used to to the conditions you will meet up on the hill.I have never ridden the Double, I have over the last30 odd years however have ridden all of it many times, and have done the length of it in one hit a few times, but that was when I was young and fit, not the old codger I am now.
Personally I thing riding from Eastbourne to Winchester is the easier route as most but not all of the slopes are kinder in that direction. wet chalk can also make the route quite lively in places, and in places it becomes mires of chalk wheel gripping slush so avoid wet days.