Go around it. If a busy trail, try to scare it off. A similar incident happened to me last fall. I was running a trail around a local lake and nearly stepped on a Copperhead. I saw it at the last second and had to leap over it. I threw a couple of twigs at it but it wouldn't move an inch... I think it was 'playing dead'.
I started to run down the trail again, but I turned around and came back after a few meters. I decided I didn't want some redneck deciding to kill it or another runner possibly getting bitten When I got there though it had already left. It apparently took off the second I started heading away.