Ran 10.04 in the morning. Started out at about 7:20 pace, then sped up to 7:10, but did not feel like going faster up until the turnaround. After that, decided to chase the 7:00 mile guy. Sped up to 6:35-6:40 pace. It felt hard, but HR was correct - 131-133. The hardness was mostly mental. I had a hard time getting my mind to engage when running alone.
Ran with the kids in the afternoon.
After talking with Dr. Jex about how the spine works I believe I have a better clue about my running economy and low sprint speed issues. I think the problem is in the disks of the lower back - they lack the resilience. This would explain low vertical lift when running (not bad thing in an of itself, but bad when even that little has to come with a strained effort), low vertical jump, overdeveloped quads, unusual difficulty in walking up the stairs for my level of fitness, relatively poor uphill performance, and the feeling of constantly running on cotton. When the spine is not resilient, the muscles have to pick up the slack to get you off the ground.
According to Dr. Jex, once the muscles learn to hold a curve in the spine, the disks have an opportunity to regain their resilience. The curve is already beginning to form. So we'll have to wait and see what happens.