I was interested in the same thing and did a bit of research and experimentation last year. What I wrote at the time:
I’ve never done rides longer than 9 hours so it’s hard to determine what my target intensity should be. Hence, how fast could I cycle for the duration of this event, which is probably 11 or 12 hours.
Luckily there’s a ballpark formula: 0.95 ^ (number of hours). A 0.5 intensity would mean you do the ride at 50% of your maximum intensity so the formula basically tells that you’ll drop 5% of your intensity for every hour of racing.
This seemed a bit conservative so I decided to adjust it and use .97. Additional math told me that I should be able to sustain 220 watts on the climbs and 197 watts for the entire ride.
I was far from in a good shape at that time but I think the calculations are still quite relevant.