"Calories in, calories out"
Stormy, I think you can solve your problem with simple mathematics, plus a little faith in your current metabolism. If you don't already, start recording what you're eating every day and counting up the calories (there are all kinds of calorie counters online that will tell you how many calories certain foods have, in addition to the nutritional information on packaging). This is a pain, of course, and there's no way to do it long term (at least in my opinion), but it will give you a rough number on the number of calories you're eating now to maintain your weight.
Then, visit a site with a "calories burned" calculator (like
www.nutritiondata.com), and plug in your height, weight, age, activity, and amount of time you spend doing it to get a rough number of calories you burn through exercise.
These two figures will give you a baseline of the number of calories you're taking in now to maintain your weight.
Then, return to your 5-mile routine, keeping your "calories in" number the same. See what happens. If the scale stays the same, you'll know your metabolism has adjusted well to the new routine and you can rest easy. If the scale starts to inch up, then you'll know you need to make an adjustment, either to "calories in" or "calories out".
However, that adjustment isn't necessarily going to be cutting back calories even further. That adjustment means that you need to change the exercise you do to something more challenging. Your body has gotten used to walking, so you need to try something else to jump start it again, to burn more calories in the same amount of time and get your CICO in balance again. I agree wholeheartedly with teck21 on that point. You're right that if you continue to walk 6-7 miles, your body would start finding THAT easy and would burn fewer calories as a result. You can't keep doing the same exercise indefinitely without having your body get used to it and hitting a plateau.
I switched from walking to jogging a couple months ago, with no ill effects on my knees as yet (and I have sensitive knees!), for that very reason. However, you could also throw a martial arts class in the mix, try spinning, jump rope, or some other more intense cardio activity. Or, you could place more emphasis on weight training.
In short, you've already proved that you've taken control of your health and weight and succeeded. So, if going back to 5 miles makes you gain weight, it doesn't mean that you should panic, or that you have to necessarily eat less, if you feel like you've already cut what you eat as far as you'd like. It simply means your body is bored and wants to try something new!