I have VO2 Max lab tested 3 times - twice at BYU, once at the Utah State University. When I was a 2:40 marathoner (St.George course, so really more like 2:43), 5 K around 16:40, it came out to be 59. As a 2:27:46 on the Top of Utah Course which is about fair (downhill fairly compensates for altitude), I had it tested at 75.9 with RER 1.00 reached at 93% of it. I had it tested another time - it was a day before I raced a marathon (same course, Top of Utah) in 2:28:42 - and I pushed to RER to only 1.02 reaching 71. My best 5 K around the time of the 75.9 test (certified road course, 1% net elevation drop, around 4500 feet of elevation) was 15:37. I have wondered for a while what was up. My form looked bad enough that my friend once told me he could recognize me from half a mile a way. I have worked on the form for a while and tried a number of different things over the course of the years without much improvement in this area.
Finally I discovered what I believe to be the root cause - a physical therapist saw a hairy patch on my back and suggested an X-ray of the spine in the L4-L5 area for spina bifida occulta. His suspicion proved correct - his opinion was that I was "brave to run with that condition". However, aside from being slower than what my aerobic profile would predict, I never had an injury that was serious enough to miss more than 3 consecutive days of running in almost 33 years (ages 11 through 44), and no more than a day in the last 20 years - the condition is mostly asymptomatic if I am careful to avoid skewed surfaces and too much track running.
Even before that I suspected that if you cannot sprint very well, yet at the same time slow down from 100 to 800/1500/mile like a typical middle distance runner rather than being able to sprint forever (e.g 60/2:12 vs 60/2:05) with the best middle-distance training, there is likely a problem somewhere in the spine. So this may be worth checking. If you can run 14:23, I would guess, though, that it is relatively minor - but even a minor problem like that could be the difference between 14:23 and 13:00. And who knows, unlike mine, yours might actually be correctable with modern science.