Experts agree that it is important to be intentional about your exercise (or training) plan – paying attention to the intensity of your exercise in particular. Two great ways to do that are by using your HR (heart rate) or RPE (rate of perceived exertion) as a guide.
Before using your HR you need to find out your maximum heart rate. This can be determined in clinical testing by medical or fitness professionals, by self testing or observation, or by calculation. The old  – [your age] formula has been found to be very inaccurate. A better formula is:
 – [1/2 your age] – [5% of your body weight] + [4 for men] or [0 for women]
For example, using my age of 48 and weight of 160: 210 – 24 – 8 + 4 = 182 bpm maximum heart rate. This is still not as accurate as any of the results from testing, but it will provide a good starting point.
After finding your maximum HR you can calculate your HR zones. Each zone (see chart) is a range of heart rates (beats per minute), a percentage of that maximum number. Each zone has certain specific benefits, and you should spend time in all the Zones. The exception to this is beginners – who should start progressively. Spend a couple of weeks in Zone 1 before you venture into Zone 2, and so on.
You should spend between half and three-fourths of your time in Zones 1, 2, and 3. This will ensure that your body is building a strong aerobic base as a foundation, and also ensure adequate recovery time after those bouts of more extreme exercise. If you’re in Zone 4 or 5 today, you should be in Zone 1 or 2 tomorrow. Many of the benefits of exercise actually come the next day(s) when your body is recovering – rebuilding and repairing the damage of exercise. That’s how it works. Recovery is as important as the exercise itself.
One of the biggest and most common mistakes that folks make (men, mostly) when returning to exercise is to do too much, too hard, too soon. It is NOT a good idea to spend all your time in Zone 4 or Zone 5.
Even if you don't have a HR monitor, you can use the same principles by utilizing RPE (rate of perceived exertion). Refer to the chart below to see how the RPE scale lines up against the HR zones.
Very, very easy
Very little effort,
Easy to talk;
Hard to Hard(!)
Very hard to #x*! hard
Uncomfortable, can't talk;
Again - the formula and the "standard" percentages yield estimations. To really dial in your zones would require testing at a sports medicine facility. But these estimates will give you a starting point and are certainly better than nothing.
(My thanks to my buddy David for his help with the html code on the table.)