The more familiar you get with your THR range, the more you can customize your workouts. And the more you can monitor your intensity. (You've seen the charts on the cardio machines, right?)
Don't like the math? Find an easy calculator online. Don't feel like stopping mid-workout to take your heart rate? Find a heart rate monitor at the store.
And, as always, listen to your body. Your heart will be the first to tell you if it needs a break, and I advise you never to let it get to that point.