Just walk. It may not be considered as good as lifting something heavy on occasion 'round these parts, but the goddess of exercise to me is walking. Unless you are disabled, you can do it your whole life. It doesn't hurt. Do it barefoot or do it in the most supportive non-grok shoes available and anything in between, but walk.
Fat, thin, (and anything in between); young, old, (and anything in between); couch potato to exercise junkie (and anything in between), just go outside and walk. Your core gets worked because you have to keep yourself upright. Your ass and legs get used. Your heart will thank you. Walk, children, walk.
If you can walk, you can walk back to health. Grok didn't go to a gym.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine