Sorry Ulster is correct. These lung measurements cannot be changed by exercise or other life choices. I don't agree you need to be particularly fit to pass a medical merely to have an absence of pathology. I really can't see what sudden intense exercise could do except cause musculoskeletal injury
Peak flow can be a third down on normal before it means much. It is a very general test, needs to be done properly, and isn't very sensitive. FEV1 is undertaken on a more expensive machine and is more sensitive. Both measure both restrictive disease such as fibrosis and obstructive disease such as asthma. We also measure the total volume of air exhaled called FVC and the ratio of FEV1 to FVC is reduced in obstructive disease but not in restrictive disease so it is a discriminator
I wouldn't wory about your reading if you have no history of lung disease