Hey i was/kinda still am going through the same apprehension about a test that you are. This is what i have learned from people on this forum and from my nutrition professor. I am by no means an expert but i believe i am very well informed. (try and talk to Fake Boobs Rule he is the recognized expert here on Drug tests). The time that THC stays in your body is highly dependent on your weight, usage, metabolism. Since it seams that you are a light user and your test is not for 20 days if you do not smoke i would say you will prob pass with no need for dilution. when it gets closer to your test date or even now if you are really stressed go to a pharmacy and get a home urine drug test, this will tell you where you are at. Now according to my Nutrition Professor drinking tons of water now will do very very little, however if you are dehydrated your body will not expel toxins naturally as fast as it would... my professor also said that green tea or cranberry juice will help slightly lessen the over all time that it takes to detox. In my situation i was recommended to take Niacin pills (if you want i will let you know the dosage when i am told) i was told that niacin works by a government official with extensive knowledge on the subject. Exercise does help alot with detox because it burns fat and releases the THC into your blood and then your urine then out. however stop exercising at least 72 hours before your test. Hope this helps