Thanks to you both. We tried unsuccessfully to use our torque wrench to get it out, so I took it to the bike shop this morning, where someone 40 years younger managed to loosen it without breaking it. It has indeed always been difficult to budge. We will make sure to lubricate it and not over-tighten it when we re-assemble. Now off to the Horsey Hundred in Kentucky, barring any future snags! Thanks again.