Very much the tale of two different runs -- miles 1-12 and miles 13-18. Weather was perfect this morning and I felt great through the first 10 miles. However during mile 10 I started to feel a some tightness in my right calf. I continued to run strongly in 11 and 12, but that calf muscle and both legs in general were not feeling totally comfortable. At 12 I hydrated and had a gel. However, when I restarted the discomfort continued -- not enough to make me want to cut the run short, but enough to make the final six miles difficult. I managed to push through with a lot more stops than I would have liked. I could run hard, but I just couldn't keep it going for more than 10 to 12 minutes before needing to stop & let my legs recover for a minute or so. I writing these notes 5 hours after the run & my legs are recovering pretty much as usual. I did not feel like I was pushing too hard in the first 12 miles, but maybe I was because the weather was so much cooler in those early miles than it has been. Not sure, but I am a little concerned as this is the second week in a row my legs have struggled after the 12 to 13 mile mark.