WALKING.
But not strolling.

To be of benefit walking needs to be brisk, such that if you try to talk at the same time you become slightly breathless. A gentle stroll doesn't result in much benefit.

The benefits to be gained are :-

It is good for stamina and cardiovascular fitness.

It maintains muscle strength in the legs.

It helps prevent bone loss.

The shortcomings are that it doesn't do much to help your general flexibility or your balance.

Research into Ageing's verdict;
Good leg muscles are vital for getting the blood back up the legs to the heart so this form of exercise should ideally be part of every older person's routine.

Running produces similar benefits being particularly beneficial for cardiovascular fitness and bone health but take care not to overdo things. Remember the No Pain, No Gain theory is nonsense. If you are feeling pain, listen to your body, because it's telling you that something is wrong! You need to talk to your doctor.