You want to try a kick-butt interval workout that doesn't mean making your heart race? I've been given zone 3, low cadence intervals for strength building and they've taught me a few things. I can probably go faster at times if I don't spin so high.... and an interval doesn't have to leave me gasping to kill my quads. Try some 60-70 rpm, hr zone 3 intervals sometime and see what you think (I did mine on the low side, I averaged 62 rpm for each set 3 x10)