I would like to run on consecutive days, but I'm afraid of getting runner's knee or general knee pain. It is healthy to run on consecutive days? And if so, how many miles should be my "upper limit" if I'm also planning to run the next day as well? For context, I've been running for 3 years and I usually run twice a week anywhere from 1-7 miles.

I run every day ~5 miles but do it barefooted on soft surface (water line at beach). I would not do it on concrete regardless how good the shoe support is. It's very important to have a well balanced body as you are putting it through higher stress, running or being a desk warrior. By that I mean symmetric use of the breathing mechanism (rib cage, muscles, etc.), symmetric use of the hips, symmetric gait. Any prior injury, medical intervention or repetitive-use trauma will affect your nervous system's willingness to load a limb, a hip, what have you.