It only gets easier.
A friend of mine who is 73 rides a 10 speed 1950's vintage bike. He is a former TdF racer and wipes me out with my 27 speed.
Lot's of folks are into single speed and fixed bicycles.
Your 12 speed will do just fine, and 21 pounds isn't too bad. My old steel mtn bike must weigh about 30 pounds or more, and I did the Ride the Rockies on it when I first started bicycling 10 years ago when I was 58.
You are off to a GREAT start and that muscle memory coming back is most interesting.
Keep it up, plan a sort of training regimen, and MOST IMPORTANTLY, be sure to give your body rest days and rest rides.
Have fun.