Trevor, I have also messed aroud with circuits similar to what you have mentioned. With on/off Achilles issues I have to get creative to keep my fitness levels up. Luckily, there are so many nice alternatives for the veteran or master’s athlete that can be used in lieu of tempo runs.
I know Charlie was not that concerned with measring heart rate for sprinters but I have been using a monitor for the past 2 years pretty consistently during my tempo/low-intensity days which I feel has been very helpful for me to ensure I don’t over do it on my easier days. My resting HR and blood pressure have gone down in the past year and I attribute that mostly to my tempo/easy days.