Some ways to increase your walking workouts would be to add resistance exercises along the way. To start chose one of these four movements and progress to eventually have all four. First, add a body weight squat. After walking a certain distance, we will say 1/4 of a mile, add 20 squats, and do 20 more for each 1/4 mile. If you are walking 5 miles then squats every 1/4 mile might be a little much so vary it depending on the length of your walk.
Another exercise will be a walking lunge. The same can be completed as previous mentioned, walking for a certain distance, then add walking lunges. A quick description of how to properly complete a lunge: step forward, a little longer than your typical step, bend and both your front and back knee, keep your weight equally distributed and be sure to sit tall, bend towards ground without having your back knee touch the ground, return to top, walk forward and proceed forward with other leg.
Your third exercise will be adding push-ups. These can be added the same as the previous two. Lastly, add a pull-up. You may or may not have access to something that will allow you to do pull-ups, such as a tree branch. If you have not access then a pull-up may not be possible but if you do then these will be great exercises to add. Be sure to choose one exercise for one walk and another for the next. Eventually, you should be able to add all exercises to add a complete full body workout in addition to your walks.