It's unlikely that a single maximum effort workout increased your fitness much. I suspect that the psychological aspect was more important. You proved to yourself that you could push harder than you believed before. Most endurance athletes lack the mental fortitude to reach their psychological limits.
You could be right, and I understand why you’d jump to that conclusion, but I don’t think that’s what happened, and here’s why. The effect didn’t last all season nor give me any boost the next one. When I slowed down my frequency of rides later in the fall, I felt my performance decline. It made climbing noticeably easier right from the start of a ride, before I felt winded or tired or like I was having to push through anything. Before this happened, I had already done a lot to prove to myself what I can and can’t do, I’ve had other times I’ve pushed myself to the limit. I don’t think this really was a single event, I think it was a boost in the middle of a bunch of rides. It was the back-to-back rides in the context of my lets-get-going early season riding, though I don’t remember exactly how often I was going. At the time I expected this ride was going to make me sore for a while and tired for a few days, like what usually happens after an extra long ride. I was really surprised when it didn’t do that.
BTW, it might be important that the sprint to exhaustion up the canyon was relatively short; it was 5 miles averaging medium steep, with spots up to 12% grade. I usually ride it in granny gear, “speed 1” on a 27-speed bike, smallest chain ring, largest on the sprocket. To follow the fixie I had to stay in speed 14 (middle chain ring, middle of the sprocket.)
This experiment very vaguely reminds me of some methods people use pump up the level of the red blood cells... I might be mixing it up with something, but it's a curious thought.
