You may have outrun your headlights by doubling your mileage on terrain your feet weren't conditioned for. Loose sand/ dirt will cause your feet to roll around in your boots causing hotspots and blisters. Foot maintenance should be performed at the first sign of a problem, although you will not always be afforded the opportunity. All in your tracking down the right path. Understand that a 14-15 minute pace is the low end of the standard. I have had to walk a lot faster, with more weight, for extended periods of time. Now back to motor boating SBG sends.