There's a local racer who did something like this, starting as a 5 in early March (2010), started riding over the winter. In four months he was already winning Cat 3 races. In fact he won all the races I entered in the middle of the summer. I am/was a Cat 3 at the time, and I got enough points to upgrade to Cat 2 that year, and he was absolutely slaughtering us. When I say "won" it wasn't just sitting in and winning, he was making moves and stuff, mainly to cover breaks, and then he would kill it in the sprint. In August of that year he upgraded to Cat 2 and got 3rd in a big P12 race. Currently he's a pretty consistent winner in the smaller P12 crits.

He had a reasonable threshold (he jokes about how low it is, just like I do about my threshold, but crucially his is maybe 70 watts higher than mine), he has a good sprint (makes it much easier to win Cat 5-4-3 races), and he is incredibly astute on the bike (i.e. he's very smart, is a good learner, and thinks very quickly). I have no idea how he learned his bike handling skills in a group but he lives in the Boston area and apparently does group rides regularly.

Another local, he fell with 1/2 mile to go in a crit in the Cat 4s (Hartford Crit). He actually crashed, he got up, got on his bike, sprinted up to the field, sprinted through it, and got 3rd or 2nd in the field sprint. After the race I congratulated him on his place because it was a great result for a new racer, and I told him that it was great that he avoided the crash. Because, frankly, how could someone crash in the last lap and still place, right? He was really upset and I couldn't understand why. He explained that if he hadn't crashed with everyone on the backstretch he thinks he would have won. Thinks?! Heh. He peaked as either a 1 or a 2, he's still extremely competitive in the P12s, picking up top 6s all over the place. The aforementioned 5->2 guy can beat him, so that's one of my comparison benchmarks.

The guys that turn pro are much stronger than that. It's pretty scary how strong they are. One Junior at Bethel got into a break with two very strong Masters (Cat 2s). He fell in the first turn, got up, sprinted back up to the two Masters (who did sit up but still, in a 90 second lap it's still a massive effort to close a 150-200m gap), led out the sprint, dropped them, and won the race. He raced for a number of years for Jelly Belly and for United Healthcare. He was 14 or 15 at the Bethel race. Another Junior, I think he was 15 at the time, won both the Junior and Cat 3-4 races overall, slaughtering everyone in the sprint. He later raced for a number of years on the National Team as part of the Team Sprint guys (they didn't do well).

I haven't ridden with many pros when they actually go hard, but I was lucky enough to watch one show up at Bethel. He raced for a domestic team, he went to the front as soon as the P123 race started. After the first lap there were five guys on his wheel, all strong 1s and 2s. After the second there was one guy left, a Cat 1, and that one guy got 3rd in the Elite RR a couple years prior, so he was literally one of the best riders in the area. On the third lap the pro was alone - he'd ridden everyone off his wheel. He lapped the hard chasing field in 8 laps or something ridiculous like that. He sat in at the back and chatted with people for the rest of the race.

That 3rd-at-Nationals Cat 1 offered to train me, to get me more fit. He emailed me some graphs as examples of what I should do (I'd just gotten a powermeter). I had no idea what the graphs meant before, when he published them on a somewhat non-public site, but when I looked at them after getting the powermeter I was floored. He was doing five 5 minute intervals, averaging 500-550 watts for each 5 minute interval. I saved the graph somewhere but I can't find it but I couldn't believe it. At the time my best one minute power was about 425 watts, and in a massively hard race (for me) with a probably-not-properly-zeroed powermeter I hit 587w. I think my real one minute power is in the 525-550w watt range but I've never hit that training. He can hold that for 5 minutes, then repeat that four more times. And he couldn't stay with the pro!

I feel pretty lucky that I'm so untalented that I'm peaking as a 3. I don't need to train a lot to enjoy racing and I have no pressure on me to do well