The 17-mile Seawall is the world longest uninterrupted waterfront path. A good option from downtown is to do the ~8 mile loop around Stanley Park, past English Bay, and an express return along Burrard St. Another option is to run as much of the Seawall as you like, using a ferry/aquabus for the return. There is good directional signage along most of the way.
Our table to the left shows distances up to about 14 miles, starting from Canada Place downtown.
We have also mapped out the 14 mile route, as well as a 5.8 mile section within Stanley Park