I based my run on my threshold heart rate based on a Joe Friel's threshold field test. Based on the threshold test, my VT2 is 163. For each mile interval, I kept my heart rate below 162. I thought that my heart rate was around 159 for the most part, but my data shows an average heart rate of 157 bpm, 158 and 157 respectively. By the third interval, I definitely needed to back off because my lungs were burning and I felt spent.
Are you familiar with the Irish Olympic runner, Stephen Scullion? He had an interesting explanation on the benefits of adding threshold training to your running.
This was my impetus to training just below my threshold heart rate. I actually didn't look at my pace at all until after my run was completed.
Thank you for your input as always, Billy. I think that I'm going to continue mile repeats using my threshold heart rate and see where it gets me. The good thing is that I have time on my side. Marathon training wouldn't begin until November, so I have three months to see where easy running four days a week (with strides on two of those days) and one day of threshold repeats gets me. But, it's nice to know where I stand with how realistically I can attain my goal come race day.