Again, thanks to all. I was suitably alarmed and swapped out the 16T for an 18T. That was a very good idea. There are some serious hills around the border between New Hampshire and Vermont, it turns out. I'm not sure I would have finished the ride without an 18T cog. (On the other hand, the downhills were deadly!)

I'd recommend the Prouty to anyone, but I seemed to be a real minority there. I didn't see a single bike without a derailleur, and those few who noticed me — probably wondering why the hell I was bombing up every hill so fast — all commented on my "singlespeed." I was too out of breath to stress the distinction.