Guys it all boils down to actually running not speed walking. If your foot lands on the heel it's called walking, when you land on front of your foot it's called running. 75% of "runners" are doing it wrong in their massively cushioned shoes, which also slow you down, but I digress. How to fix it? take of your "running" shoes and try running on asphalt/concrete/pavement bare foot. If you run correctly you won't feel a thing - if you speed walk you will start feeling pain in your heels after 30 seconds. Afraid of this test - do it on a sandy beach first. Read Gordon Pirie's "Running fast and injury free" and run like a boss.