I'm 68 and back into cycling for three years now after a 35 plus year lay off. This year I'll have a little over 4,500 miles in, many metric centuries, and can hang with spirited B groups.My goal for 2021 is 5,000 miles and some A group riding. Not repeating the great advice so far, I'll add at "autumn age" that sleep and recovery are where the fitness gains are made. Look up a devise called Whoop, it's a game changer for me allowing much better fitness gains. Other devices do the same thing. I'd add once you are green lit by your doctor and if you have a smart trainer, don't be afraid of structured training as you will do a test in the program and the program will determine how hard to push you. A smart trainer adjust the resistance to match your current ability. You can pick a plan and your target date for your half century and all you have to do is follow it. Now here's where a device like Whoop comes in as it tracks your sleep and heart rate variability HRV. So each day you get a recovery score and target strain for the day. A low score indicates you are not recovered enough to take on a lot of strain. I use an app call Sufferfest and their All Purpose Cycling training plan. Yesterday was a hard interval workout. Today my recovery is low so I'm pushing today's activities in the plan till tomorrow. It's hard to go by "feel" as I have had great recovery scores and didn't feel like training but once started I crushed it. Also vise versa, pushing when the data says not to and that created set backs. With all that being said, yes add streching and strength workouts too. Don't need to be a lot and apps like sufferfest allows you to add these to their plan. You will automatically achieve some form of fitness just by riding, it's really how far you want to take it. Do not be discouraged when you start indoor riding. No coasting, inflexable bike tends to make iit challaging at first. This is why I like structured training indoors as you can get a lot of gain out of a little time spent in the "pain cave".