Getting fit by using your heart rate as a guide.
This will prevent you from overdoing it, and on the flip side, it will make sure that you are training enough.
See the target Heart Rate Zones training chart here:
How do you determine your maximum Heart rate? (MHR)
This is the maximum recommended number of times your heart can contract at any given minute. An easy, relatively accurate way to determine your maximum heart rate (MHR) is by using this age-predicted formula: 220-age=MHR (for men), 226-age=MHR (for women).
Determine the Zones by using the formula found here: