First, subtract your age from 220. If you're sixty, you'll get 160. That's your max. Now take 60 percent of that number. You'll get 96. That's 60 percent on your max. You initial fitness goal should be geared toward staying above 96. As you get stronger, you will be able to go beyond, toward your max. of 160. Remember to use your age when calculating. Heart rate monitors are not that expensive. Invest in one and kick your aerobic fitness in high gear. Your ultimate goal is to truck along at near your max. for extended periods of time. Got this one marked down really low. An old model but is very good. It's a Garmin 305.
Gear up and get started!