I had never heard of this before I came across this article. But, I can attest that even when I was in great running shape, something similar I used to do busted my ass. I'd do 15% incline at a pace ranging from 3.0-4.0 for 30 minutes to an hour. Ask anyone who knows me and walking fast is not my thing. I stroll through life, so a 3.0 pace is fast for me; 4.0 is extremely uncomfortable. But, it can work you especially hard when you add a little incline. Try it. And if it's too easy, take your hands off the heartrate holds and swing them like you're out in the wild. If you aren't sweating, breathing hard, and begging for that clock to get done, I'll be impressed.