they both wore a watch and one of their HRs averaged a ridiculous 115-120 at a very slow jog; about 8 minutes per km pace. after every 800m or so they had to walk for a while. they were not fat, their respiratory systems must just be really weak. but now i understand why people at the park don't run faster, it's because they can't sustain even a relatively slow pace for more than a
few 100 metres before they get into HR zone 3.