You shall know a good ride, young grasshoppa', when you have one.
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At 22, I was riding 500 to 600 miles a week in early summer to race. By 32 I had a sedentary desk job, by 42 middle age spread, by 52 high blood pressure, by 53 a heart attack and two stents. Happily at 64 now, blood pressure is normal, weight is down, cycling is back in my life, and life is good.
If you don't already have one, get yourself a decent blood pressure meter with an arm cuff. Make a morning ritual, daily record. Relax and gain awareness of changes. Follow your doctors advice, all of it. Adopt a heart healthy diet. Discuss changing drugs, lowering dosages if you have side effects.
btw, sanchan is just trying to help you. Clogged arteries can be more subtle that one would think. In hindsight, I wish someone had more bluntly warned me what comes next when I thought that all I had was high blood pressure. Putting myself at risk of a heart attack is one of the stupidest thing I've done (so far). High blood pressure is the wake-up call. You might get to push the snooze alarm once if you're lucky. I got away with it. I got a second chance. Not everyone is so lucky.
Once you get into shape, stay in shape. Don't slide backwards over the winter. Listen to your body, it will let you know when you've overdone it and when it's time to kick it up a notch (when you're bored). Add distance a little at a time and the mileage will pile up as if by magic. An ancient cycling tome said each day of riding adds a brick, but a day skipped removes a brick from the wall you are building. 58 is old enough that you might need more recovery time (days off) than when you were young, especially after a long ride. Sometimes a short easy ride is better than skipping a day, sometimes you just need a day off. And the day will come, sooner than you think, that 12 miles has become a short easy ride.
After you've knocked off some weight, you will round into better and better form, making it progressiively easier to lose the next pounds. You may hit a number of plateaus that take time to break past. Persevere. You're off to a good start with your rides so far. The simple fact that you know how many rides you've been on and how far you've gone is a good thing which you can expand on. Keep a log of your rides, cumulative mileage, weather, wildlife seen, how you felt, weight that morning, diet. Put lots of detail in there, and make it personal. New P.R., best time for a particular circuit or hill climb, longest distance ridden to date. It all helps you keep motivated. It may not seem important as you write down what now seems obvious and trivial, but on your 63rd and 68th birthdays, you will have fun looking back and seeing your progress over five and ten years. Or...forget all that and just ride and enjoy being in the moment. Whatever floats your boat.
Cycling is a long-haul endeavor. You will learn a lot about your body, other people, and nature.
The beauty of forums like this one is there's always something new to learn from other posters young and old.
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"If you can walk away from a landing, it's a good landing.
If you use the airplane the next day, it's an outstanding landing." -C. Yeager
Don't know if Chuck Yeager really ever said that, but the same might be said about riding.
It's all about getting back in one peice and being able to go again the next day.