I do almost all my workouts on the mill.
I see a few problems with this:
- 30 second recovery intervals are very tough to do accurately because it'll take 15-20s just to get up to 12mph depending on where you start.
- 45 second intervals are also very short and you'd need to be accurate to do it right, and that's hard to do on a treadmill.
- 12mph on a mill is ok, but ONLY if it's a high quality commercial mill (4 or 5 horsepower). If it's not, it's just not going to happen. It might say 12mph, but it won't be.
Most of the workouts I do on the mill are longer (min 800m) and have longer recoveries (min 60s). The one exception is hill sprints, in that case I get it up to speed, hop on, run like mad, then hop off.
Good luck!