is Concord/Walden small enough to find everything on foot or would we need some kind of a tour?

Rent a car. You can't do it on foot and I don't think there are any tours.

FWIW, Walden Pond is pretty much just a glorified swimming hole.

There's a train from North Station to Concord center.

www.concordguides.com
http://www.freetoursbyfoot.com/getti...oncord-boston/

There's also a bus tour via allBostontours.com

We were able to tour Concord/Lexington via on/off trolley. We were able to visit Alcott house but I don't think it went to Walden Pond. I liked Fruitlands which would require a car.

Some of the historical sites that are part of the Boston National Historical Park have admission fees. If you are interested in going inside of these site, you can purchase a senior access card, (requirements are 62 years old and a us citizen or permanent resident) for $10. its good at all national parks and national historic sites; one time life time fee; and the card holder can have up to 3 guests. Once you buy the pass, you do not pay any admission fees at these sites. If you are interested look at the national park service website for Boston

<FWIW, Walden Pond is pretty much just a glorified swimming hole.>
So what's your point? It's a pond, and I, for one, found inspirational the fact that people were using it, to fish and swim and so on, and that it had not been turned into a cordoned-off Tribute to Thoreau.

There is a very low key display near the entrance to the park, across the street from the pond itself, that recreates the tiny cabin he built for himself. Interestingly, the display notes that he was escaping the increased development of the area at the time, getting away from it all before it became impossible to do so, in his view.