I'm sorry for being terse in my initial reply; I was in a hurry. What I truly meant to point to was the idea that Zvi keeps hammering home that there is no binary of 'safe' and 'not safe' actions. You asked whether walking would reduce risk relative to being stationary, but the question James answered was essentially whether being outside was 'safe enough.' (I want to note that this is James's first ever LW contribution, so it's an understandable mistake.) So, given your initial question, I don't think you should have updated on James's answer at all. Being outside offers lower COVID risk than being inside, because there's more exchange of air. A windy day is in turn probably less risky than a day when the air is stagnant, for the same reason. Presumably you could simulate windiness by walking, biking, or jogging – the main open question for me there would be, like, if you're directly behind the other person, maybe all the air they breathe out is breathed in by you just seconds later, which seems bad. This comment is also not an answer to your question, because I think I'm just restating the model in your initial post. The important/interesting part of your question is to what extent. Hopefully someone more informed than me comes along and can give you hard numbers :)