Here are some suggestions. Please, though, if you have real physical concerns, are overweight or over 50 years old, consult a physician before doing anything strenuous. You'd be surprised how far you can walk on the level, but add some incline and everything changes. Don't forget to consider the condition of the trail. A perpetually wet trail, or a poorly maintained trail is a more difficult walk than a dry dirt road.
Vertical rise is the elevation change during the hike.
If you start at 1,500 feet elevation at the trailhead and top out at 3,000 feet on a mountain summit, your vertical change is 1,500 feet, though you may have traveled several miles. Also, trails are not always a steady incline, but descend some during the hike, before climbing again. Milages below are round-trip. Level - 1 to 3 miles . Accessible by nearly everyone.