Since being so jubilant at my recent successes, I checked what sort of times professional riders are taking for similar distances.
For the Olympic Time Trial last year, Wiggins averaged ~32mph over a slightly shorter, but I imagine much tougher, course. For the final TT in the TdF, he held ~31mph over a slightly longer, presumably also harder, course. The slowest tour rider (153rd) finished with a respectable ~26mph average.
Consider my feet well and truly planted on the ground again.