But you must be doing some cycling around that as well? Is the session over an hour long? - even on my ERG controlled trainer and looking at the metrics in Training Peaks I wouldn't be able to match that gain from a single session per week... there's no doubting structured training gives the best bang for time invested - if I could do it with just one session in the pain cave that would be a great breakthrough.
I cycle, but not so much any more since PBP2015. I was averaging about 5,000 miles a year until I finished PBP and my ability to cycle 600km type distances was undoubtedly better than it is right now. However Wattbike classes have allowed me to keep in great shape without losing endless weekends to endless Audaxing (I may just exceed 2,000 miles this year).
If I was still Audaxing AND doing Wattbike classes then I have no doubts I'd be fitter, faster, stronger, at 55, than I have ever been in my life. I reckon my local TT times would be the same now as they were 20 years ago.
Extreme endurance fitness plus power would be a wonderful combination and cycling would be so easy. However my life is heading elsewhere and I can't be as selfish with my weekends now.
Just a word of warning though. My Wattbike results haven't come easy. It's bloody tough, as painful as anything I've put myself through, but the rewards come quickly. I fully understand why people wouldn't want to do it.
To the OP and the original question. It's a distinct possibility, actually unavoidable if you put the hard work in.
FYI. My MMP (Maximum Minute Power) has gone from 312W to 393W when my starting position was off the back of PBP2015 and 2 years of prep/qualifying so I wasn't exactly starting from scratch.
The Michael Moseley BBC documentary (about HIIT training) discussed the genetic component of improving fitness. The results of exercise/training do not impact everyone the same.
There's a Bell-Curve of response to exercise. On the extreme left are people who simply don't improve as a result of training (Michael Moseley is down there).
On the extreme right of the curve are the elite athletes, the people who respond exceptionally well to training.
You can find this out by recording your training results over time or (as Moseley did) having a genetic test.
Add a natural ability, like Chris Froome's (high altitude) lung capacity, to a super-responder's genetics and you have a Tour de France winner....if they train hard of course (The very best have all physical advantages PLUS the mental ability to put themselves though daily torture sessions for years on end).
I seem to improve quickly but I also seem to "lose it" very quickly.
Some of my friends seem to be able to "store fitness", to jump on a bike after a long layoff and remain fast. That's not me. I lose it if I don't use it, and fairly quickly.
It may be depressing but some people just won't ever get faster. They WILL get healthier though...just no Gold medals.