Yeah do the excercise but not on the 2 days before your test, THC metabolites are stored in your fat cells so you don't want to be releasing them at test time, also, drinking lots of water/fluids on the day of the test will help, but anything before that is pointless as THC is fat soluble not water soluble - water simply will not clean it out of your system.

I was a chronic user and passed a urine test on day 22 of not smoking, get some home tests and test yourself they are cheap. Just remember not to use your first wee of the day for your test, drink lots on that day, and if you don't have enough time you could always just "forget" your ID and ask to go back in a couple more days, as this looks way better than a refusal or a fail, but still slightly suspicious.