I just did my first ever 40k TT. It wasn't an official race, just me on an isolated trail seeing how I might do, so I decided to take the plush M5 rather than the faster but much less forgiving M1.
The experience only confirmed what I've always known, but I'm glad I did it. The first few miles went fine, occasionally reaching a cruising speed of 27 mph. But after mile 5, I experienced a big drop in power output, losing about 1 mph. I thought it might have been caused by a sudden headwind, but a glance at my Garmin removed all doubt; it was definitely a power loss. The only good news for the remainder of the TT was that my fatigue level didn't increase any further, as I feared it might. In the final two miles or so I managed to keep going at ~25 mph without any noticeable increase in lactic acid or running short of oxygen. In fact, I'm now convinced that I could have pushed myself just a tad harder for an extra .5 mph, but didn't realize that until the very end. When I dismounted and tried to walk the bent back to my car, I felt dizzy, a bit nauseous, and my hands were shaking. I became aware of a thumping sound and thought it might be a jogger on the trail behind me, but it was the sound of my own heartbeat.
So, no real surprises here. As has always been the case, it seems I'm just not cut out for any sort of endurance sport. Still, I did well enough to win one KOM on a seven mile segment and narrowly missed wining another one of equal length. That will give me the incentive to try again.