An object at rest will stay at rest unless acted upon by an outside force, just as an object in motion will stay in motion unless acted upon by an outside force, according to Newton's Laws of Motion.
Guess what? You're an object! Surely you've noticed that the longer you lay on the couch, the harder it becomes to get up. And what about the opposite? I can't tell you how many times I've heard people say "once I get going, I'm good". Yes, because of physics. This isn't just YOU that feels this way. It's a law of motion.....
Here's the hard part. When it comes to physical activity, that outside force has to be YOU. It's literally up to you whether or not to stay active. You are what you do.... Laziness promotes laziness. Activeness promotes activeness. The choice is yours. This is true for you, no matter what your limitations may be. All the way down to being wheelchair bound. If you stop practicing the motion of standing, you stop being able to stand. It may become more difficult for you to perform a certain motion, but the less you do, the further you get from easily being able to perform a motion until eventually you can't even do it safely at all.
No one can keep you moving, but you. No one can keep you active, but you. No one can make you become sedentary, but you. The choice is yours. Unfortunately, doing nothing is easier than getting up and getting active. It's no easier for me than it is for you. Newton proves that! The same laws apply to everyone. We are all going to have different limitations. It's what you do with what you've got that makes a difference in how you live, act, and feel. Hang on to what you DO have. Don't let the things that ARE impossible for you to do, keep you from doing anything at all. Maybe you don't have the ability to ride a road bike, but you do have the ability to get some low intensity aerobic activity in. You do what you can with what you've got, or you do nothing at all and fall even further away from being any kind of active.
The choice is yours. It's all up to you. What are you going to do about it? Complain about it? Blame it on your age? Or a college injury? Or an old surgery? Or are you going to do what you can with what you've got to work with, so that you don't lose that too? Add life to your days. Add quality to your life. Keep yourself active, or get yourself active if you've become sedentary. Make the most of this life we have. Do what you can to be sure you enjoy it to it's fullest. No one wants to simply exist. Everyone wants to live! Make it happen. Be your own outside force. No one else can do it!