I was in Friston this eve and its got very wet all of a sudden, still great, but might not be ideal for 4-5hour ride.
The top (the SDW in effect) should be okay still, but I’ve not been up there so not entirely sure.
As said above, if you keep roughly to the SDW you’ll be okay. Dont be afraid to drop off the top, either North or South to explore, and maybe save an hour or so for Friston on your way back.