Speed + Hills = Treadmill "Trades"

From this interesting web site comes a new treadmill workout: Trades. After a warmup, get into your easy run pace and do the following:

• 2:00 at 6-8%
  
• 3:00 rest at level to 1%
  
• 2:00 level at a speed roughly equaling your hill effort
  
• 3:00 rest at level
  
• repeat "until your brain explodes or your legs fall off, whichever comes first"
  

At my easy pace of 6.3 mph (9:31 pace), I did the hills at 7%, which was doable for two minutes but very difficult. The strange thing is, I almost couldn't run fast enough to equal the same effort; I did my last two speed intervals at 8.5/9.0 mph (7:00/6:40 pace) and came close, with a maximum HR of 177. I did four sets and the whole workout with warmup and cool-down came to 6.0 miles.