In the end, after much consideration and flip-flopping either way, I decided to give the Olympic distance a shot. This was partly due to C's encouragement, and partly because he's going to be unbearably proud of himself as it is. I've got no time goals for this one (though under 5 hours or so would be nice!) -- I'm just aiming to finish. Even if I have to walk that last 10km. But I'm really not planning to walk it. At least not the whole way. ;) Secondary goal: not coming in last. Second last is not a problem.
To make things easier for myself, I rented a bike. My own bike is a mountain bike, and while it is very faithful, and gets me all over the place carrying all kinds of things, it's not really the best bike for a street race. I was looking into buying myself a race bike, but apparently this is a bad time of year (who knew? I kind of assumed summer was biking season), and all the pros buy theirs in January or something. Anyway, the bike stores are basically out of stock of anything remotely in my price range. Hence the rental.
The bike they gave me is lovely. This is it here. It weighs next to nothing -- it's really quite incredible. I took it out for a test ride tonight, and was whipping along like crazy. I'm sure it'll take a few minutes off my time. It's also a really lovely shade of blue, that pretty much exactly matches my helmet. Not that that matters. Really.