I LOVE THE SDW - at least, I love the bits of it I've ridden. I've only done sections close to home, and have no experience of the whole hundred miles, but would wholeheartedly agree with ThreeSpeed's observations.
See my gallery for pictures of the Fareham Singlespeed Mafia doing the bit of the SDW closest to us. We get out via the Meon Valley Railway Trail, so can ride a 50 mile off-road loop from home, taking in a nice chunk of the SDW (including its highest point), without having to drive anywhere.
I quite regularly do the Warnford to Butser section, taking in Beacon Hill, Old Winchester Hill, Wether Down and Butser Hill - with a return via the long bridleway down to Denmead, then Bere Forest and back on the railway. Butser is the highest point on the SDW (270m, 888ft), but there are some really tough climbs through Sussex - which I've not sampled myself. I tend to find that the bits us SS'ers need to walk are also walked by the gearies - unless those gearies are race-fit and highly skilled! However, if doing the whole thing I would gear down considerably - and probably allow two days for a first attempt.
We're doing the 50 miler again as a club ride, taking in the aforementioned SDW section, on May 26th Bank Holiday. All are welcome to join us. More than half of our little group will be on SS. The details will go up later this weekend on the forum at www.fareham-wheelers.org.uk.
One day I'll get round to doing the whole SDW. One day....
"Gears are like eyes or testicles. Having more than one is an unnecessary luxury." Elvis A. Presley