It was a windy morning and I planned to run an HM today.

At the moment, I was worried more about distance than speed:
"Will I actually be able to RUN 26 miles in 2 months?" I
hadn't even run an HM since going BFT. The bottleneck had
been the calves for the past months. I did a lot of stretching
and managed to do full flat-foot squats. It helped but not as
much as I expected. And so far, I was able to push 11 miles at best.

Then I read in the Dicharry book (p163)
         Let's think about a runner with inhibited glute max
         strength. To run a certain speed requires a given amount
         of force. Now the glute is a major player in delivering
         force to the extending hip, but if you can't activate
         it, force has to come from somewhere. One of those
         places is the calf. ... Because they can't get the hips
         to store and release this elastic energy, their calf is
         forced to do way more work than it was designed for and
         breaks down. No amount of stretching, mobilizing, and
         ankle strengthening will fix this scenario.

Was this what happened to me?

Steven Sashen said something similar in his blog article
"Does barefoot running cause calf pain?" and suggested
         ... Again, the solution isn’t to hit the weight room and
         improve your calf raise strength. It’s to LIFT your foot
         off the ground (instead of pushing) by flexing at the hip.

The wind was strong against me and there were not many
people on the trail on this chilly morning. I had a steady
pace under 8:20 for most of the first 8 flat miles. Then
there were the hills, my speed dropped to 8:39. Back onto
the levee, I got the tail wind but was not energetic enough
to speed up much, although I did finish stronger than
started: 7:57, 7:59 and 8:03 for the last three miles.

"Activate your glutes" was repeated numerous times in my
mind during today's run. In the end, my calves were painful
but not debilitating. This is where my focus should be from
now on.

For the record, I did an HM at an average pace of 8:16
(total 1hr 48min--new PR).