Running is harder than a Cathe interval w/o I believe. My heart rate stays up there in the higher levels for a longer sustained time. One thing I have noticed, and maybe some of you more experienced runners can tell me, is that when I hit the upper level of my training zone and actually above on say Cathe's IMAXs, I am very aware of my body taking in as much air as I possible can but when I run and my heart rate gets up there and stays up there, after a little while, usually about 1/2 mile, I will... what I call "settle in" to my run and not be breathing that hard at all, at least I don't seem to be.