First some food for thought: a teacher at the Daily Kos (Teacherken) wrote an interesting diary concerning national standards for public school education. The diary can be found here: decreasingly diary.
I have to digest what he said and think further.
Dus7 has asked some questions in a comment and I shall attempt to answer them.
Walking paces for distances vary widely from walker to walker. At a typical walking division of a marathon (here I am using the Lake Geneva Marathon (26.2 miles, or 42.2 km), which is on roads but features some stiff hills), the times typically vary from 5:15 (12 minutes per mile) to 8:40 (20 minutes per mile) with 7 hours (16 minutes per mile) being typical. World class racewalkers can go as fast as 7 minutes per mile (the world record for the 50K (31 mile) is 3:36, which translates into 6:58 mpm, a pace which most runners can't even dream of meeting for this distance. Typical road 50K runs are won with slower times than this.
I typically walk marathons between 5:13 and 5:35 (12 minute pace) and 50kms (road) in 6:20 to 6:45 though I go faster when I walk with bent knees (illegal in judged racewalking). So my typical 20 mile training walk is at a slower pace than that and usually takes 4:30 (if I am going well) to 5:20 (tired, just strolling) and perhaps slower if I use an off-road trail. A National class racwalking friend of mine takes his "easy pace" 20 milers in 3:40; an Olympic walker may take 2:40-3:00 for the same distance (in training).
I do not listen to music while walking.
I don't stretch prior to the long ones, but I take yoga 3-4 times a week (and adore my maternal, matronly, middle aged yoga teacher).