Could not find this listed and it wouldn't let me add it as an activity. Can the moderator maybe add this as an exercise?

Because of how close it is to walking/running, I usually substitute that exercise for it. Remember though that in FitDay PC you can create your own exercises, in the same way that you can create your own foods. It won't give you, as far as I know, the ability to do different levels of the same exercise however.