My Mum and Dad who are younger than you and older than you respectively and also from NZ don't have any trouble getting around on their visits to Tokyo. You should really venture outside of Narita - it would be like going to NZ and spending the whole time in Wiri!
Given that you might be day tripping it and it's your first time (?) in Japan, I would recommend trying to bite off small pieces - don't try to see everything in one day. Just looking at the Imperial Palace or just looking at Asakusa can easily fill up most of a day. If you're not a big walker, then the hop on/hop off bus tour might be a good idea. My parents are quite active - they go for long walks in NZ - but they somehow end up walking about twice as much in Japan as a good day in NZ. In 30 degree heat (and summer hasn't really started yet!) that can be a bit draining. here's the hop on/hop off bus link: http://www.skybus.jp/?lang=en&no=&bno=