Depends how strong you are. There's 10000ft of climbing, the worst of which comes at the (Eastbourne) end of the ride.

It would be tough. Some big old hills I have only ever used gears to do it

I've never tried to do end to end without gears, but I've ridden quite hefty stretches of it on a singlespeed.

At my level of fitness, to do the whole distance without dying of cramp I've always had to push a few hills anyway. I reckon it'd slow me down to have no gears, but it isn't the end of the world. You'd just have to follow the Way of the [s]Mountain[/s] Downland Turtle I guess. 🙂

I heard you can do the double in under 24hrs on one if you are fit enough...

http://www.bikemagic.com/event-features/south-downs-double--on-a-singlespeed/6370.html

http://www.bikemagic.com/gear-news/neil-newells-on-one-inbred-29er/6374.html

it's ok if your used to singlespeed.

3 of us did it; 2 of us geared down a bit.

there are a few hills to be walked but not significant.

very do-able, I've done it a few times with mates SSing
I'd swap rear sprocket adding a couple of extra teeth on the back from whatever you usually run though.
Yes there will no doubt be a few walks in there E->W Butser hill is a killer, a few others in there too but not more than a mile or so's walking out of a 100 miles

I did about 75% (West to East direction) of it SS and it was ok - however the last 25% is the hardest bit IMO and I was pleased to have gears when I did the whole lot!

Done it twice all the way singlespeed
Once geared.
I preferred singlespeed, however the geared ride was a wet and windy one
There is a flatish bit in the middle of the ride where if you were racing gears would make up time, not to owrry if you are enjoying the day..