A stress test
is not a test
of your physical fitness, it is a test
of your cardiovascular under a work load. The work load is set to do just that, bring you HR up to a load level.
They will hook you up to an EKG (maybe more, I don't remember) and start you walking at a slow speed, say 2 to 3 mph. They will then watch your heart traces (EKG) and slowly increase you load, most likely by increasing the incline. They will ask you how you feel, you are not expected to work to exhaustion.