high intensity interval training. sprint 1 minute, walk 1 minute. rinse and repeat 5-7 times and done. theres a million different apps for it too.
that was jsut an example. you can tweak the duration of run/walk or do run/jog or just do it on a bike or elliptical. running is the best method if youre already really lean
theres also something called a HIT routine which is high intensity training. intensity and volume are inversely related, so think of it as lower reps for heavy weight type of training, aka strength training