Thanks for all the good insights and advice.

It seems like ensuring and maintaining comfort on the bike is going to be a big issue. I found in my first foray into distance on the fixie that I couldn't use all the usual tricks in my bag to keep the contact points (hands, butt, feet) comfortable. There's no coasting and standing to stretch out on the pedals. And I found myself seated a lot more than usual. Turns out there's a narrow range of cadences where I like to stand and on the FG I either had to slow down or use the terrain to slow down to achieve those cadences (as opposed to just throwing another few cogs on the fire and then standing up). I think in time I'll get more comfortable standing when turning a higher cadence. What other tricks do you folks use to maintain comfort on the bike?

I'm sure in time I'll figure out the track stand (turns out that, though I can track stand on a geared bike forever -- or at least for a light cycle at a major intersection -- I'd always initiated the track stand with a preceding coast and breaking that habit is a work in progress). Unclipping and clipping back in is a pain and saps energy. On the truly long stuff, I just don't want to spend the energy getting in and out of pedals. Sounds silly, but ride 750 miles sometime and you'll see!

Thanks for the comments about the bike set up and fit. I had the guy who built the bike (from stuff I'd been acquiring for years; after finally realizing that I had no time to build a bike, I turned the job over to a professional) fit it exactly to the bike I've successfully used for long rides. I mean, it's exact. I'm wondering, though, whether I might want to shorten the stem a touch so I'm less stretched out. That might be more comfortable given the inability to generate comfort in other ways.

The bars probably have to go. They're the 26mm aluminum Easton EA30, which is otherwise a fine bar, but aluminum can be pretty buzz-y over long distance. The AL IRO frame definitely needs as much dampening as possible, so there might be an upgrade in the near future to an oversized carbon bar to help dissipate some of the road buzz. I'll probably also add aerobars (now there's a look you probably don't see much of in the SS/FG community!) for the flat-ish 1200K, if that's a "go," to add more riding positions.

I should probably also go with some fatter, lower-pressure tires. The 23C Conti 4000s are a sweet (albeit expensive) ride and provide flawless handling, but even the most comfortable 23mm tire (which these are not) will pale in comparison to a 25 or 28.