I’ve done MK to London a few times. It can be a bit bleak and relentless at times Berkhamstead is a nice place to stop, good canalside pubs.
I walked Watford to London once in a day. I won’t do that again, I can tell you…33 miles of tedium, and we had to break into the zoo to complete it (t’was dark by then). As it turned out, breaking in was easy, getting out was another story.