I read on paragonrunning.com before it closed that JK and Kyle Heffner said that anaerobic repetitions could not bring a person to a different fitness level, but only bring their existing fitness level to a degree of race-worthiness and preparation to handle the effects of lactic acid accumulation.
I tried to do some high--for me--mileage during the summer after many years of injury. I started racing again and began with a time right outside my Pr. My Pr is very bad, though, so the result for me is unwanted.
My question is what can be done to completely and dramatically change fitness levels? I understand it takes months and years of mileage and other work, but I always enter a season feeling a lack of fitness. Someone like Sileshi Sihine, when unfit, can still easily break 14:00 for 5K whereas I am way off that at my very fittest. What can be done to bridge this aerobic gap in terms of specifics and whatm in your experience has led to some really dramatic improvement for someone who's stalled at a poor performance level?
I am very interested if you could reply and thanks for your help.