I took 15 hours first time. Someone did it three times (Winch - East - Winch - East) recently in 30 hours. If I had set my mind to it and not boobed quite as much as I did, I reckon I could possibly have completed in perhaps a bit over 13 hours.
I commute ten miles each way on a road bike (and have been for some years) and I cruise at about 18mph on the flat in windless conditions. I do seem to go up hills faster then those who go past me on the flat.
On my recent two runs along the SDW over the course of two days (Thursday and Friday each time), our rolling average was 7.1 mph for the first one and 8.1 for the second. If you are trying to complete in a day then (it's blindingly obvious really) stoppage time is key. When I did it solo a couple of years ago I didn't stop cycling for the first six hours other than stopping to take water on and let it out (and the enforced fettling session when I fell off).
If completion is highly desirable then having a number of spare tubes is essential. I even carried a spare tyre just in case. There are a lot of flints just waiting to slash your sidewalls open.
I understand from people I've encountered en route that the two major reasons for failing to complete are punctures and getting lost. Getting lost is really, really dispiriting, especially if you descend any distance.
It's good to be well prepared for an abandonment. The Harveys map I used doesn't extend much beyond the route itself, so if you're using that then prepare beforehand by marking on it routes to train stations.
There is a fish and chip shop opposite the station in Eastbourne. A very welcome sight.
Shout if there's anything more you want to know.