Not technical, so can't answer the question in the OP.
My final success (at the third serious attempt) on the SDW was last year and was the one time where I hadn't been able to undertake any training rides. My riding last year consisted almost entirely of my 10 mile commute to work (and 10 back) over one slight pimple of a hill (Coombe Hill near Kingston on Thames). Other riding would have consisted of bimbling around with kids and quite possibly an FNRttC to Brighton - hardly endurance building stuff.
I don't amble on the commute, but I'm not exactly quick by the standards displayed on yacf, I generally roll along at 18mph.
For the day of the SDW I started early and fed and watered myself respectably well, until near the end when I missed one of the water taps. The last three hills were much harder than the previous 27 (guess at the number) and I really slowed down. What I don't know is if they were objectively harder than the previous 27, or whether my fatigue combined with lack of water or quite possibly a mild bonk just made them seem that way.
So, in answer, lots and lots of endurance riding may not be the only answer to completing. Doing it with more than one gear would probably have helped quite a bit.
