Stephen Wolfram (of the Wolframalpha.com fame) has a treadmill attached to one of his workstations [0]. I've read claims that walking 1-2mph takes no mental effort. Lets say 4 hours of walking would net you about 5 miles. That's a decent effort for the day and you could still sit the other 4 hours out of your 8 hour day.

[0]: https://writings.stephenwolfram.com/2019/02/seeking-the-prod...