Take food. Your feeding strategy will be key. I did it in 2 days in June and took me a couple of days to recover – and I'm not unfit! Lots of hi-energy, slow release stuff. And feed well the day before.
If you search this site there's a previous thread which had a link to a site where all the water points are marked. I then marked them on my OS maps and took them with me so I didn't miss one. If you do the distance is quite long to the next one. Dropping off the downs at any point to get food water is a major time-consumer. Go self-sufficient if you can.
Oh, and enjoy. It's a wonderful ride. If the weather's good the views are fantastic and you'll feel well pleased with yourself once you've done it :-)