Huge change in volume and quality will cause massive change or burnout.
When I was 18 I did 10k miles. It was pretty much the worst year I rode - DNF something like 40 races, finished just one.
When I was a bit older (mid 20s) I did something similar, massive miles and racing, a much higher level of competition, and I had the best year of my life. It was an absolute world changer, to experience significantly faster riding, to comprehend huge and previously unfathomable expectations (and then try and meet them). I'm not an athlete naturally so this was a huge change for me. Stories linked below.
Story:
http://sprinterdellacasa.blogspot.co...kermesses.html
Pro sitings:
http://sprinterdellacasa.blogspot.co...messes_28.html
Pictures:
http://sprinterdellacasa.blogspot.co...kermesses.html
Also there's a concept of being able to try radical approaches to the same problem (i.e. before you wouldn't even try it). For me it's gearing - if I usually use my lowest gear on a hill, what if I use my biggest? It's a lesson I learned as a kid:
http://sprinterdellacasa.blogspot.co...ic-and-me.html
Finally, a lesson from the above link - if you can get fast on the flats, you get fast overall. Climbing fast isn't where it's at, not at the beginning. It's the fast stuff on the flat roads that open new worlds.