The original post never did call it a "race", and that's one reason I didn't assume it was a straight-through type thing.
If it IS a "race" (I'm thinking like the Silver State 508 or Texas Time Trials or Race Across Oregon or that sort of thing), it depends a lot on your expectations AND on the time cut-offs that the race organizers have.
On the Texas Time Trials, the 500-mile option (800k) has a 48-hour cutoff. You're basically riding two 400ks back to back.
You can go out on these races and suck at it, and that's okay. But if you hit the time limits, you wind up as a "DNF" on the records rather than a slowpoke. And in that case, you want to do a "24-Hour Race", not a "XXX-Mile Race".
From the description, it could just as easily be something like RAGBRAI, though.
If this is unpaved or offroad, that's a whole nother animal, tho.