How do you get your exercise? Do you agree if you don't use it you'll lose it?

Since my back surgery I walk 2 miles in the morning before it gets real hot...if I don't I have back pain all day then I hit the trail in the evening!!! Do you think outside walking is more strenuous than a treadmill? I use to do water aerobics but they lowered the water temp and it's too cold for arthritis people!!! How do you get your exercise?