Thanks. I appreciate all the specific advice I'm getting here.

I'm not particularly obsessed with exercising, I just want to make sure I'm keeping my body in decent shape. A sort of loose near-future goal was be able to jog for 20 minutes, since being able to go faster for extended periods of time will actually be useful to me getting around the city.

Right now I do a lot of walks. If I don't take the bus, which I often don't, it's an hour walk from the train station back to my house at night. It doesn't elevate my heart rate but it at least replenishes my supply of appreciate-the-outdoors-warm-fuzzies. But I can't maintain a continuous run/jog for more than 10 minutes.