In 2015 I walked 1,200 miles during lunch and on weekends and my HDL was crazy high! I now prefer bike riding but my HDL is over 10 points lower (still good but not great), and I attribute the better numbers to walking.
Teddy Roosevelt thought that a healthy man should be able to walk 50 miles over three days and in under 20 hours . . . meaning the clock starts on day 1 when you start and then stops when you finally quit that day (with breaks WHILE STILL ON THE CLOCK as being permissible). You do this again on day 2 and 3 too. I was considering it and did walk 16 miles in 4:40 a few Saturdays ago (and that included a 15 minute break at the halfway point). I kept myself occupied with several podcasts
I didn't continue with days 2 and 3 but I think I will later this year.
As far as calorie burn, I always used 1/2 a calorie per lb of bodyweight per mile. Meaning a 200 lb person will burn 100 calories every mile or 200 calories every TWO miles. If you are willing to devote an hour a day (catch up on your podcasts
) at 4 MPH (15 minute miles), then a 200 lb man can burn 400 calories from just walking . . . that's nearly a lb loss every week just due to walking.