You are going to feel similar fatigue, especially over a 9-10 hour duration. The altitude causes you to burn more carbohydrates for fuel (ie shifts your fat burning zone to the left and then some) so even at the lower wattage you will deplete your glycogen stores at a similar rate as you would at the higher sea level FTP. Maybe you have a bit more time to depletion at 850kjs/hr vs 1100 but for me at least, it has always been very easy to hit empty in higher altitudes. So that will cause fatigue and limit your performance and something to be very careful of with your pacing and nutrition strategies.

Also the central fatigue from pushing near your limit will be similar at 230w as it does at 300w. Your body still needs to pump out the same amount of stimulus to the sympathetic nervous system to keep you thumping along at the same high HRs. You will be in and out of similar amounts of lactate with the inevitable punches on the course. The actual muscular demands of 230w vs 300w are a small factor compared to this other stuff. You might feel a bit more springy after prolonged FTP efforts at 10000ft vs. 0ft but don't count on it. I can guarantee that if you go into the red for too long you will feel much worse than usual.

I think thats a good way to start, using the percentage adjustment you mentioned above. Most times I find I can get a little closer to sea level power at z2-3 effort than z4. Again, that is probably an effect of flirting with lactate and the more profound effects of oxygen debt at altitude. Its an individual thing though and the best way to find out is to test. If you are there ahead of time like 5-7 days out you could do a 3-5 hr ride with a 10 minute effort at z3-4 (altitude adjusted wattage) every 45-60 mins. Pay attention to how it feels, your HR and fatigue build up in the later efforts. If 235 is a max effort in hours 3-5, you know that you should keep your efforts lower on race day. If you feel strong on the last one you can probably use climb at 90% of that FTP for your long race day climbs and recover well.

The power meter is super useful at altitude, you just have to figure out your pacing numbers ahead of time not on race day. If you can't get any real rides in up there beforehand, just start super conservative and gradually bring it up with 235w as a cap. In a 10 hr race there is lots of time to make up for a more conservative start.

Hi, we can help you out here as we commissioned some work on this topic for a future calculator (on cyclingapps.net) on altitude effects from all the published work to date. Actually we already built a draft model which works with fairly high precision and I ran some numbers for you......

At 12,500 feet...........
Oxygen pressure is 60% and temp difference vs sea level around 25deg cooler
Aerobic loss is 25% but there is a 35% gain (reduction) in aero drag
The overall effect is approx a 10% gain when riding on level ground (less riding uphill due to reduced aero benefit) (not adjusting for temp)

At 10,000 feet................
Oxygen pressure is 67% and temp difference vs sea level around 20deg cooler
Aerobic loss is 18.% but there is a 29% gain (reduction) in aero drag
The overall effect is approx an 11% gain when riding on level ground (less riding uphill due to reduced aero benefit)

To work out your anticipated pace based on power, send us (or post) your FTP at sea level and your anticipated average altitude (over the whole ride) and the anticipated race duration and we will send you back your optimal race pace average power....however this is only an average you still have to decide how are when to vary this according to minute by minute conditions.

ps. as we are in draft version....If anyone has different data I'd be interested in the source and correction. We also have data on the exact benefits of training at altitude, but I'll skip that for today.

best of luck with your race!