I stayed for a few nights in Chiba with a friend of mine who is a talented martial artist and health and fitness expert.
Per his recommendations, I'm going to try out cutting all carbohydrates six days out of the week (with one cheat day) and fasting one day per week. I cut the carbs two days ago - I already didn't eat sweets, but moving off rice and bread (sandwiches) was somewhat limiting at first... in a convenience store with 400 items for sale, there's literally only 20 or so I can eat.
But that's fine. Surprisingly, I haven't been hungry at all... sometimes I think I should eat, but my stomach hasn't growled at me at all. I'm probably eating too much fried chicken (quick, fast, cheap, widely available in Tokyo), but I'll adjust as I find more options. Tonight I bought a small salad and a big can of tuna for breakfast tomorrow.
My buddy gave me a pair of spare Vibrams he had (same shoe size) and recommended some slight walking and posture changes. Also, some more stretching/strengthening/bodyweight exercises.
I'll work those in over time - I'm quite busy right now, so don't have the attention to devote to a whole new fitness regime immediately. But I'm doing the no carbs/cheat day/fast day right away. I'll post updates in a while with how it's going.