This goal has the following fine print specified by fryhole:

Any sort of exercise that I can record on Strava counts. This uses something Beeminder calls "effort weighted time," meaning that walking counts normally and anything else counts double. At this early stage, that means I could get away with 75 minutes a week of activity, which is probably too little. I'll keep it this way for the rest of January and then decide whether to bump it up. [Update: I did, to 4 hours per week, which is still only 2 hours if I'm doing anything but walking.]