The original plan was to do a 24-hour cross-country ski race on this same weekend, but of course it was cancelled due to COVID-19. With all outdoor options being shutdown one after the other, I retreated to my garage and settled on the century row, since 24 hours without logistical support seemed unreasonable.
Here are a few things I learned that will probably be of little use to me in the future but might be of interest to anyone contemplating a similar endeavour.
The Body
To be clear, I went in absolutely unprepared. I wouldn't recommend it. My biggest month this season was 133km, in January. My biggest week was 45k, just before Christmas. My longest continuous row was 1h/15.xk, in November. I'd been a bit more diligent on the snow so I did have a decent endurance base, but it's just not the same. I truly struggled in the second half, in particular the last 30-odd km. Presumably, had I done more HM/M/50k's in the months prior, I would have suffered a bit less.
The worst part was of course the butt pain; excruciating, ever increasing, debilitating butt pain. I started out with what I thought was a decent thickness of bubblewrap on the seat. After a while I added a folded towel, then an other, and another... By the end I was sitting on 3 inches of cloth and plastic, but it was too late. Again, maybe some longer rows would have hardened/desensitized that area abit, but evidently I should have used more padding from the start.
Some surprising pains of note: my abs, and my finger nails (index and middle finger). Both parts of the body are still sore 48h later, when everything else is nearly back to normal, apart from a couple of blisters that haven't quite subsided yet.
The Mind
I was rowing for charity, live on Facebook. Those two things got me across the finish line. Having music helped, as did my family dropping in once in a while to check on me, but if I hadn't been motivated by people watching me and "paying" for it, I would have stopped after 60km. A purely solitary row would have been too much pain and not enough reward.
I did have some goals in mind (sub-7h, French age group record), but when those went out the window it was difficult to find new ones. Silly games like counting 50 strokes with my eyes closed kept me focused for a minute or two, but when things got rough it was mostly a matter of shutting the mind off and not thinking too much. All the while remembering why I was doing this in the first place.
The Numbers
7:35:09.0
2:16.5/500m
12 energy bars/gels
5L of sports drink
1200€ raised so far
About that average pace: I didn't anticipate how much time would be lost drinking/eating/wiping my hands etc. I actually rowed at 2:10/500m or faster most of the time, but stopping/slowing down to refuel put a huge, demoralizing dent in my real average pace. I kept rowing while I drank from a camelback (every 5k or so), and mostly pulled on the handle a bit to keep the monitor from shutting off when I had to stop completely to eat or whatever. But even short rests add up to minutes, probably 20 minutes in fact when you include the one potty-break at the half-way mark.
One thing I've been wondering is whether you can "cheat" the monitor having someone press the buttons instead of slowly paddling. It seems to me that after a while the timer stops, and stays stopped until you row again, as long as you keep pressing "units" or "display". It's surely within the rules, but I would think you save a lot of time like that compared to pulling at 2:30/500m for 30s. Am I wrong?

I think I've rambled on enough, now. I'm happy that I didn't quit and I'm grateful for the support I got before, during and after, but it was one of the most physically unpleasant experiences in my life. Now it's back to regular, confined training for the forseeable future. Yay?