I started back at week 1 day 1 of the couch to 5K program-- that means running a minute and walking a minute and a half, repeat eight times total. That should be easy peasey for me, except for one thing-- I hit the turbo button. I did seven of the runs a third faster than my admittedly tortoise-like normal pace, and then the final run at two thirds faster than normal.
Verdict: I can do that. Heart rate monitor sez it's pushing the edge, though, and I certainly couldn't do much more than a minute of the fastest pace. I could do a little bit more of the 1.33X-normal pace, but I'd probably hit the upper bound of acceptable heart rate after no more than two or three minutes.
Interestingly, I went back to counting steps per minute. I'm doing approximately the same number of steps, just longer strides.
I'll probably do this again from time to time just to mix it up.