Follow the 12 Mile Creek! Basically just ride out of Port Dalhousie on Martindale Rd, over the QEW, untill the start of the Merritt Trail (across from the stoplights at Erion Rd).
Option 1) Simply follow the trail, keeping the creek on your left (East). This will eventually get you to Short Hills Provincial Park, but there are many trails along the way. Many of the new trails near Glendale Rd were made by Liberty bikes for their summer race series. The red arrows mark their course.
Option 2) Do the reverse of the Lake-to-Lake race course until you get to Brock U. See map at:
http://www.mapmyride.com/route/ca/on/port%20colbourne/666547187111
A great place to start is at Glendale and the 12 Mile Creek. There are many ways to go from there and there is a pedestrian bridge across the creek. This is at about the 39 KM marker on the above map.