Mid-Rehab Stress Test

On Monday I took a stress test on the treadmill at my cardiologist's. The results left me wondering. Your maximum heart rate is calculated using 220 minus your age. I'm sixty-two which means my max rate is 158 (supposedly the fastest that my heart can beat). For Cardiac Rehab they gave me a target rate of 120-128 which they want me to hit and maintain for twenty minutes on two different machines. That's 75% and 80% of my max respectively.Here's my question: during the stress test I went to 170 heartbeats a minute and held it (and didn't die, thank God!). Doing the math backwards, does this mean I have the heart of a 20-year-old? (170 is 85% for a 20-year-old, 100% for a 50-year-old).