I too have worked out a 3.6 mile walk route from my house that is not only pleasant and safe, but is rolling and flat terrain. A great walk for those days when i don't bike for one reason or another.

I think if you read up about the techniques of fitness walking (as opposed to just walking along casually) you will find you can increase the exertion level and also the upper body workout using those techniques. Of course I think Nordic pole walking is actually a higher upper body workout...but I find I get the workout I myself need by using the fitness walking techniques. You can use your arms more efficiently too.

When I just bike alone, my knees and hip joints are not as strong and flexible as when I alternate walking and biking. This is quite important to me at age 53!