Would love to hear from any cyclists who’ve taken a prolonged time off the bike and then restarted cycling - really interested to hear how quickly you regained fitness and how you found that journey. Aware there is going to be a huge amount of factors at play here but from chatting to a few riders it does seem FTP can be regained fairly quickly (6/12 months). For some context, I’m riding again after an 8-year hiatus during which time I’ve eaten and drunk a fair amount/ done zero cardio. I’m 26 and previous FTP was 302 watts at 56kg however I weigh 85kg now.
I haven’t taken that much time off, but at only 26 it isn’t like you are likely to have chronic health issue that would limit your ability to get into shape in a relatively short amount of time. I would guess you will regain at least 90% of your FTP in 4-6 months of decent structured training. I also wouldn’t be surprised if you didn’t get to a higher FTP.
An ftp of 302 at 18 and only 56kg suggests that you have quite a bit of genetic potential and your body has now fully matured. Honestly, I think your biggest hurdle will be trying to balance some weight loss with training. Your body composition may change nicely with the addition of regular exercise, but any sort of aggressive dieting would work against your fitness goals. Moderation is the key.