yesterday my left calf tightened up at the half-mile point, and there was no way to stretch it out or run through it. it was an actual injury. i think i may have changed my stride thinking about "barefoot" running and not landing on my heel. today i just ran normally; i didn't try to control my stride in any way. i also took the first half of the run slow, taking it easy with my leg. the first 1-1/2 miles was 18:34; i felt fine at that point, so did the 2nd half in 16:06. i do need new shoes, but more importantly i need to just let my body take care of business without letting my head get in the way.