I'll spin out on descents, but then I don't pedal on descents anyway and I mostly use the breaks so that shouldn't lose too much time. The ups, well on my old SS I could get up Dulwich College hill, the hills up to Crystal Palace and the hill up the outside of Greenwich Park (Crooms Hill?). none as steep at the Elbow, but then I walked up that anyway last year with a geared bike so how much worse can it be
On the flatter sections, on a 42/16 gearing, I should be able to cruise nicely at 15-18 mph. I'm much fitter this year compared to last and I completed the 100 miles last year geared so I know I can do it again this year. Challenge therefore is do I try to go faster this year, or try a different challenge? Well, I want a SS so it kind of makes sense to get the bike for the DD and just see how it goes - if it falls apart badly there is a broom wagon that will be nice and warm