What to do about drug test?

As part of a "rehabilitation program" I have to go through for a charge against a tobacco pipe found in my car (long story, but would have tested positive for pot residue), I have to take a random monthly drug test. I do not smoke marijuana and this random test would be a cake walk if it wasn't for my daily environment. Almost daily I work with and around someone who smokes continuously (he's 100% legal in doing so) and sometimes I'm around when he and his card-carrying friends are smoking. I'm concerned that I will fail the tests continually considering I am stuck in these circumstances indefinitely, or at least for the period in which I have to test. Added to it, I have read that increased exercise and activity heighten THC levels, which I feel makes it even more damning for me because my job usually consists of a good workout-constant movement and exercise. What can I do??