if you are doing cardio, you must sweat. this is the simplest rule for knowing if your workout is effective. its an easy rule to remember and works no matter what level of fitness you are at.

if you want the single best cardio/toning workout, i cannot recommend the rowing machine enough. if you think this is a lightweight cardio workout, my guess is that you have never tried it. the first two minutes are a joke...in ten minutes you won't think its a joke

otherwise running can't be beat. i do a trail marathon a month on avg. trail running is the shit.