I'm assuming this is a Nuclear Stress Test. If so, they will first give you an IV and then inject a chemical that will go through your arteries. Other than a little cool sensation the first few seconds, you will feel no different. In other words, piece of cake. After that, they will normally lay you flat on a table and take images, similar to taking a CT scan. This gives them images of the chemical flowing through your arteries at rest.
After that, you will then perform your stress test. You will be on a tread mill with ecg wires connected to various locations on your chest. I'm sure based on our situation they would have a cardo doctor near you. They will slowly get your heart rate to a target rate based on the following formula: 220 - you age X .7. So for example I'm 47 my target rate would be 220-47X.7=121 bpm. Now in your case they may go less than that. When you reach this target rate they will inject more of the chemical into your IV while you're running. You will feel nothing different from that. When you reach that target rate they will normally ask you to let them know when you think you have 60 seconds left in you before you can't take it. When you reach that point and the 60 seconds is up they will wind you down.
Shortly after that they will have you lay down once again to take more images of the chemical flowing through your arteries after they've been stressed from the tread mill.
All this will show them how your blood flows through your arteries both at rest and after stress. The test will also provide other useful information such as blood pressure at rest and at stress, beats per minute both at rest and stress, etc.
Overall, no big deal.
God's speed to you!