I worked in Tokyo for a while. If you don't take cabs, you end up doing a fair amount of walking. There are a lot of trains and subways, so there is a limit to how much walking you have to do. Even if your origin and destination were both right next to a subway station, however, you could expect to do a lot of walking underground, since there is a lot of stuff underground in Tokyo around which subways have to be routed.

I think walking is great, and frequently I would figure ways to skip a subway line by walking a bit more, but I thought I'd mention this just to give a more accurate impression.