Quitting before the start date: is the non compete enforceable?
The non-compete is unenforceable unless the employer has provided you with material information that you can use to the detriment of the employer. Even in that case, the non-compete as outlined in your description seemingly exceeds the legitimate interests of the employer (see art. 2089 of the Civil Code of Québec). This means that the non-compete clause would be curtailed in court because the employer cannot prove that his business (or market share) encompasses the area depicted in the clause.
The reason of being of non-compete clauses is to protect the employer from a potential misuse of information the employee obtained pursuant to his employment, such as trade secrets and information about clients. The fact that you are a fresh graduate suggests that you are dealing with just some typical intermediary who really has no "trade secrets" and who is not providing you with key information, let alone for a job that is scheduled to start five months later.
Non-compete clauses that are overly broad contravene public policy in that they would singlehandedly exclude civilians from the labor market and disproportionately impair free trade.