Can someone tell me are these workouts best performed on a treadmill or track? I cannot find the answer to this anywhere though I’m sure its stated and I just am not seeing it!
It is personal preference, with some consideration of how safe you can sprint on a treadmill. I prefer a track for any Zone 4 and above if I can access it, but open road is just fine for me. Zone 4 is OK on a treadmill, and I have trouble nailing any Zone 5 on a treadmill.
But, to directly answer your question, there is no author preference on the venue for these workouts.