Why would you follow specific rules for maintaining (or obtaining) your health and fitness anyway?
Am I talking about the Purpose of Life?
Sorta but not that deep. Here's what you want (if you're like me):
live long
make memories
be useful
have fun
matter to someone or maybe a few special people
be remembered
Knowing that, here's the health and fitness goals you want to achieve:
Don't Die Early - seriously man, this is important.
Maintain Independence (strength, energy, mobility, balance, conditioning, etc.). Here is your focusing question: Will you wipe your own ass on your last day? Sorry for the visual, but that's a goal of mine. I don't want to put my kids through watching me slowly die.
Brain Health. Maintain cognitive ability. Getting dementia is not just bad luck. There is a lot you can do to promote brain health and much of it also promotes cardiovascular health. A two-fer.
So, here's Rule #1 for Health and Fitness After Age 50
#1 - Lift Heavy Things
Now first, heavy is a relative thing. what I mean is that you need to engage in some form of resistance training (and there are many ways to do this) with enough intensity to break down muscle fiber and increase your strength.
Strength Solves All Problems.
Maintaining independence is all about building (or rebuilding muscle mass).
Here's the message I want you to take away.
Building muscle, getting stronger, engaging in some type of resistance training is not a nice thing to do. It's not "you should do it". It's a must do. It's the most important aspect to your overall conditioning program.
Walking is great. Biking is wonderful. Playing golf is fun. It just ain't enough. And it's not even close.
So Rule #1 is to actively combat sarcopenia (which is muscle wasting from disuse and it is likely happening to you) by lifting heavy things.
A corollary to Rule #1 is Carry Stuff Around.
Carry things in your hands while walking. Hoist them onto your shoulder and move around. Hug them to your chest. Push a wheelbarrow full of dirt. Load up a backpack with weight and just walk around. Drag a tire across the ground. There are many ways to "move under load" which is a functional application of Lift Heavy Things.
You want to be a fun grandparent?
Better be able to carry a young child without blowing a nad or sucking wind.
Remember, Strength Solves All Problems.
Here's DIY instructions for Farmer's Walk weights. One of the better (and very simple) applications of Carry Stuff Around is the Farmer's Walk.
Make these weights. And then carry them around.