I'll be visiting London for the first time and, as such, can't gauge
what's walkable. This will determine whether I need to pick up 7-day
Travel Cards or just work with day offpeaks. I'm a decent walker,
but
not ready for primetime marathon walking.

[..snip..]

You will be fine with all of the examples you gave and indeed might be
surprised how little time they take to walk. From the Albert Hall
right across zone 1 to the Tower Of London on the other side is
probably less than five miles.