Judging by your age/stature etc., I would guess walking is the best option. As a runner, I've found that the best way to build endurance/speed is not to do long runs over and over. I tried that for a few years and never got stronger/faster. I started doing intervals and hill repeats and I got stronger, faster, and more resilient.

Basic interval training is this: walk at a normal pace for 5 min (warmup), then go hard (walk fast!) for 2 mins, walk slow for 1 min, go hard for 2 mins, walk slow for 1 min, and repeat and repeat for 20-30 mins. Basically you are training your heart/lungs/muscles to go at a hard pace. You could probably find some plans doing a google search. Like I said, this has done wonders to my running abilities.

Then, I would mix in some hill repeats. Walk up hill as fast as you can, walk down slow, repeat, repeat. Again, you are training your body to work harder than it normally would.

If you just walk the same speed over and over, your body gets used to that and won’t “improve” as there is no reason/incentive to improve. The way to build muscle/strength/speed is to trick your body into thinking it needs to be stronger than it actually needs to be. Doing intervals will stress your muscles telling them they need to grow and get stronger. Regular walks tell your muscles that they are fine as is and not to expend the energy to grow.

Hope this helps!