Honestly you don't need any more noobie mixes than a few weeks - the rest is up to you. The issue is that the people you're playing against in noobie mixes ought to have very little experience and so they won't press you to improve in any meaningful way. I don't claim to speak for the noobie mix organizers though so it's up to them.
The way *things* used to work, is you'd go into a website like www.tf2center.com and play there. Make a few friends, play more centers with them (we called them lobbies HEH) and then make a team with them and play next season.
Or, alternatively, you could make a thread in the recruitment section and try get on a team which may be forming for next season already, or get picked up by a team in need, or ask to be a sub and get to play every once in a blue moon, and you'd get to hang out with players who were making their way through comp tf2. You could also do the same for a UGC team - I'm sure there are plenty of UGC teams currently that need starters and back-ups.
Also to shamelessly self-promote my youtube channel www.youtube.com/medimarx has lots and lots of stuff geared towards teaching players how to play TF2 somewhat. As folks who've done my mixes can likely confirm, you'll get the same there as you will from me live and in person - I'm a sound board basically.
You could also check out: https://www.youtube.com/playlist?list=PLD098F81F2CA74F00 which has some really classic videos from some of the best folks that've ever played the game.