I slacked off on cycling several times in the past - the las time was in the later 80s/early 90s when I took up sailing. Coming back I found it took bit of time to get back into shape and I picked up knee problems but my weight was under control. That was mi early 40s. I kept the riding up but the intensity slacked way off as the knee held me back - unfortunately 30 lbs also got added. For the past 3 years I have lost the 30 lbs and greatly improved my skills. My speed went from an average of 15.5 to 17.5 but it has taken some serious training - not just casual riding.
Welcome back - you can do what you did before on the bike - you can do more. We have a few serious racers in this group and thier posts are pretty inspiring.