Hi Lillian. As others have said, please don't worry about the walking test. There's no set target they expect people to achieve. They just want to see what you can manage at the moment, so that they can determine how best to help you improve on that.
I've been on two PR courses and the session groups were very mixed in terms of age and physical ability. Other health problems such as heart conditions, arthritis etc., are taken into account so that the exercises are tailored to the individual. A lady in a wheelchair attended my first course and by the end of the 14 weeks, she was able to walk 15m unaided.
The educational side of the class is beneficial too and I learned quite a lot about my condition and how to manage sudden shortness of breath etc.
Hope all goes well for you tomorrow.