Basically I am curious about why it's fairly common to do something like 10x400 at mile pace (250% of distance; granted, this is more of a sharpening workout) but also 30-40min of T (which is theoretically 60min effort) whether continuous or intervals. Besides time constraints and overall volume - obviously 60+ min of T would be an insanely long session - why is it safe to do a higher relative proportion of a more intense pace? Does it have something to do with rep length? I am probably not articulating myself well, I guess I'm curious about where to draw the line (i.e. can you do 5000m+ of 5k pace reps? what about 10k pace?) and how to know workout equivalencies for each target zone (30 of T = ? of M; = ? of CV, etc)