How much walking

I'll be traveling with a friend who has MS and can't walk very far,
maybe a few blocks before she has to sit down and rest. We're
considering a trip to Japan, but I'm concerned about the amount of
walking that will be necessary, particularly in train stations. I've
been told that you might have to walk a mile or more when changing
trains - is that true?

Any recommendations on where/how to travel with this kind of
disability? She doesn't want a cruise, for various reasons.

Thanks!