Hi
I did the SDW in late October last year but I walked from home (near Salisbury) to the start in Winchester which made the whole trip about 130 miles. This took us 6 days at our moving average of about 3mph. We did B&B's all the way (which we prefer) so the trick is to get them spaced out. We did daily mileage of 24.5 max and min 16 but that was totally down to the accommodation available.
I would say the way to do it is Winchester to Eastbourne as the scenery just gets better and better. This does mean doing the Seven Sisters / Beachy Head on the last day which we ended up doing in a hurricane. (brutal!)
A great hike - have fun. Let me know if you want to know anything else.
Johnny