The plan was to run 12-14 miles today, but my calf started bothering me a lot more during my run today, so I cut it down to 10 miles. I stopped about 4.5 miles into the run and massaged my calf out for a little while and that helped. No more pain for the rest of the run after the massage.
I found a good sized knot in the muscle just above the insertion point of the calf on the tibia. It is the same problem that led to a stress fracture just before the St. George Marathon in 2007 except on the opposite leg. It has almost always been the left leg that has caused me problems in the past, but ever since I switched to more of a forefoot strike I have had more problems with the right leg.