I am not sure whether to assume some stuff like it will be cool enough so you do not overheat or that you know not to overhydrate or underhydrate, etc. If your weight stays the same or you gain weight on a 200, you drank too much. Anyways, different topic.....read "Water Logged"...
I can ride at around 190W for a 400K Brevet if it is not hot. Hot and me do not mix well anymore. I have to back it down in the heat. When I was young, different story with heat.....bodies change.
Riding in a fast group matters a lot. It saves tons of energy (calories). You will have something like an overall 5500 kCal deficit to power your ride assuming your glycogen stores are all topped up at the start. I am burning at least 70% fat at those power levels or more but close to 0% on climbs because that is where I am pushing it as hard as I can. Assuming 6% grade average climb (might be too steep), you have around 32 miles of climbing and 32 miles of descending and of course 136 miles of flats. These are wild guesses knowign nothing about your route.
Those 136 miles of flats might only take 5:45 in a fast group saving you maybe 45 minutes in time.....pretty huge.
The 32 miles of descents could be 50-60 minutes if you are fearless, jam over the top, and tuck.
How much time is left to climb? Assume you only need 3 lightning quick bottle changes (4 sets of bottles since you started with a full set....it is cool out, right) and that takes only 5-10 minutes (big assumption). You have maybe 4:10 to climb 32 miles of the assumed 6% grades and assuming you are not heavy, that would appear quite possible but these things can seem easy on paper or online calculators these days. If the climbs are really hot or you are not fueled for a hard effort on the climbs? Probably not. No, correct that. NFW.
Last and biggest challenge is the required energy. Not your FTP. How you fuel that power for 10 hours. Big difference compared to a Century or even a double metric (200k). Where will those 5000 or so extra calories come from? No way to absorb 500 cal per hour at the power level needed and therefore, it gets down to fat burning capability that will make or break it. Do you have a fueling strategy? What will you use to fuel? What has your experience been with various sources of fuel? How many calories can you take in per hour? 300 per hour is a lot. 200 should be easily achievable if it is not hot. So, you need to be able to burn 2000-3000 from fat, 1600-2000 stored glycogen, and 2000-3000 from what you eat on the ride. (I do not remember your weight, so, these are illustrative examples). How will you carry these powders, gels, whatever you eat? Given that the climbs will be ridden very hard and will consume so much glycogen, your efficiency on the remaining part of the ride will determine your success and that includes being extremely quick at stops like in and out. You won't have time to figure your fuel out at rest stops. Will someone be handing you already prepared bottles? If not, you need to carry your powder on your bike and mix it.....figure this time. Fat burning percentage varies based upon our training, %VO2 max used, and of course power output during the ride. I have estimated mine at LT1 by starting a ride with glycogen topped up and riding at LT1 until I bonked and frankly, I think this is good training to burn fat but that is just me. I have also done other things to maximixe fat buring capabilities. For reference, I can go 125 miles at that pace (around 190 watts) without eating or consuming any carbs but I would guess most riders need to eat if they go beyond 50-60 miles because they are burning so much glycogen at relatively low power. If I were you I would try to figure fueling out and develop a strategy around it. How much can you consume per hour after hour? This is a whole big experiment. I stay clear of fructose and that means alot of commercial energy drinks. I often use straight modified corn starch (100% amylopectin instead of a mix of amylopectin and amylose) although maltodextrin with some BCAA thrown in is also good and cheap and helpful and works good for me....100-150 cals per hour keeps me going at MY pace. If you can train your gut to handle AND absorb 300 cals per hour and you do not have central fatigue cropping up at that distance and you have good wheels to follow and it is cool....maybe you can crack 10 hours. Go for it. It is possible. I think it is a long shot but don't listen to me, I tend to prepare for the worst case scenarios and am happy with it turns out good.
Sorry if this was overly pedantic.....I would say let it rip. Worst case? You bonk and/or throw up. Rest for 30 minutes and consume carbs and you will feel better. Accept the results. Learn from them.
