Master the Yamanote Line.
This is key to get you around the city. Simply this line goes around in circle, stopping main area of the cities. There is 29 stations, 60 min to go around. Fare is b/w 140 yen to 200 yen, depending on how far you go.

Look for the Green Sign, color is key. If just one stop, that means you may have a chance to walk saving 140 yen and get your work out. Also, you can keep going around as many as you like if you don’t have any plan for the day.