Effects of High Intensity Training on Vo2 Max

Hi all,
I am an 18 year old male currently training for a triathlon. I started cycling since last May and really have found a huge passion for riding my bike over running and swimming. I was always an active kid but never trained intelligently and to this degree as I am now. I have really only done a lot of long slow distance training, staying at around 65%-70% of my max hr. Been training a lot; around 13-14 hours on average since July. I did my first FTP a few months into training and hit a bit over 4w/kg. I still have a big issue getting my hr up because my legs give out when the watts get high. Overall, since May I have seen some significant gains in my overall fitness. Anyway, I just wanted to give you all some of my background.
In a few weeks I will start to implement high intensity work into my training as I build towards my race. I am trying to keep my training as disciplined as possible and aiming to resemble a 80/20 polarized training plan. I have worked hard for a while to build a big aerobic base and I can tell the effects of my base training are coming to a plateau. Now this makes me wonder how much of an increase does the build phase increase vo2 max when you have a solid base of fitness? How many years of consistent training does it take to reach your max potential in vo2 max? I really want to see how far I can go in cycling if I continue to train consistently for the next few years. Thank you all,