Not to worry. This is a common problem. Usually the doctor who ordered the test is part of the team giving the test. Make sure you tell your concerns to the doctor or the staff at the test site.
There are ways to modify the test in such cases. You can still get the test results you need without harming yourself. When you arrive at the test area, the doctor or staff will explain what to expect. You can ask them to lower the speed (intensity) if needed. Report any pain or symptoms during the test.
If worst comes to worst the test can be stopped early. There are other less physically stressful tests that can be done. They are usually more expensive and harder on the patient. The exercise stress test is still the simplest, least expensive way to assess your heart function.