Update: Have since done 2, 200K (125-mile) rides on the bike with no changes to it. The first was pretty flat (2700 feet of climbing); the second wasn't (9700 feet of climbing).

Here are a few observations from the limited data:

• A lot of the discomfort in the butt disappeared on the hillier ride. Just getting up out of the saddle put an end to the rear-end problems.
• Thank god for two brakes. You've got to be certifiable to run only one on a hilly route. Braking on the down-hills is a ton of work and alternating hands is key. Any thought about minimizing clutter or weight or preserving some kind of track aesthetic is out the window when you're coming down a hill on which you've previously hit 45mph on a CX bike running 32mm tires. (If this doesn't sound like work to you, go do it for 10 hours and check back in with me about how you feel!)
• Braking that much takes a toll on your upper body, not just your hands. Much moreso than on a geared bike. If you're going 30mph but your bike (and you're body!) want to go 45mph, all that potential energy has to go somewhere.
• You can get up some crazy-steep s*** if you're determined enough. I walked nothing, but had to tack on one nasty hill.
• You climb a lot faster on the FG than on a geared bike, all things being equal. Keeping that cadence up is key.
• Similarly, you don't spend as much time jacking around on the FG bike. No lollygagging after a climb or putzing around. My rides felt much more "efficient" than on a geared bike.
• You pay a lot more attention to the terrain on the FG bike than on a geared bike. You're "reading" the terrain to anticipate where you'll need to stand or brake or where you might comfortably grab a drink or a snack. Hills that before looked like they were one pitch now reveal themselves to have little benches where you might grab a stroke's-worth of rest or steeper spots where you've got to really focus on making sure that top crank arm gets around to six o'clock.
• Riding distance on a FG bike can be solitary. On a hillier route it would be difficult to stay with others. You'd climb faster than many and descend slower than all.

400K (250 miles) this Saturday -- doubling the longest distance to date and keeping the bike set up the same. This route has something like 16K of climbing on it, including one hill that I'm certain I'll have to walk. We'll see how it goes. More data to follow!