I queried privately about this subject with Rob and Graham,
and they were kind enough to reply in detail! We thought it might
be helpful for others as well, to see their answers compiled into
one post, so...
Following are some of the questions I asked and their answers,
which they have so graciously given me permission to share!

*********
What is a charity walk?

And for me, the real question was what _isn't_ a charity walk!
I was getting confused thinking that the "walk" part was like
Walk/Runs for Charity in the US, where lots of people start
at the same place and time and end up at a finish line somewhere.
So, thanks to Rob and Graham, I now know that a letterboxing
charity walk is an event in which clues for a specific number of
letterboxes which are hidden for a certain amount of time along
a specific route, and the people who purchase the clues go
a-hunting at their own time and pace.