Walking gets you in shape for walking.

A walking course that has some uphill inclines will help get you in shape for more uphill walking. Walking trails with a rough, broken surface will help strengthen your ankles and knees for walkiing on (you guessed it) trails with rough surfaces.

If you plan to carry a load on your hikes, then start carrying a load, too, if that is feasible.

Just do it as often as you can, and work your way up to doing as much walking as you have time and energy to do. I walk about 2.5 miles a day all year long, but it's easy and normal for me because I walk to work and back. Most people have to look a bit harder for their walking exercise than that.

Luckily walking is a very low impact workout, so your body can tolerate it very well even when you are out of shape. Good luck.