Is walking one or two miles a few times a week comparable to going to the gym?

I’m in my 20’s and I eat moderately well. I don’t get a lot of exercise – I don’t play sports, run, or go to the gym…but I do walk. About 3–5 days a week (depending on the week), I’ll walk a mile at the least, two miles at most (depending on the day).

I enjoy walking. Is it good exercise? I don’t expect it to get me “ripped”, but is it an acceptable way to stay in shape? When my doctor is asking me if I get exercise, can I tell him yes?