like to total 20 miles each week. Currently I'm walking a little faster than a 3mph pace. I'd like to get that up to 4mph.

That's more than I average. I walk 5 or 6 mornings a week, typically 2 or 3 miles at a shot. I do push the pace -- over 5 mph, sometimes up to 6 (as fast as I can walk before I have to break into a trot... which hurts my knees...) That is about right -- I've been averaging 12 to 15 miles a week plus my 2-3 hours per week of calesthenics (....and 15 to 20 hours a week of manual labor... ugh)