yeah, its pretty inexact
i do 'obviously aerobic runs' - say marathon pace plus 45 seconds, for 20 miles+, to train fat burning (and 'leg toughness' if you like)
intervals/hills as fast as i can maintain, three minutes intervals, for LT
pace runs for, er, pacing
that's it - good aerobic base + high LT = maintainable pace
none of this fancy zone stuff
same on the bike