Not far at all. Takes all of 30 seconds to cross from the main terminal exit to the garage. If this is the first time you have used EC you may have to sign at the exit booth and show your credit card. The return process is generally quick, but, much depends on how busy the return lanes are. I would leave DTD at about 4-4:30 and hope not to encounter the rush hour traffic and I would take 417 to the airport.