Per Don Y:
For me, the advantage to a machine is that I could rest a book on
the handlebars -- not possible to read while walking otherwise
(the book bounces around too much).

Have you tried music?

Probably not for everybody, but for me it introduces a feelgood factor.

Books/TV, OTOH, just distract me from the effort - not a bad thing - but
music actually enhances the sensation.
--
Pete Cresswell