Since much of the lower areas are in and around marsh lands, please do not run after a rainfall. It will take a couple days to dry up.
The Simics Trail, as it's known to mountain bikers (or Lake Laurentian Loop for the hikers), is a great, rugged challenge for those that enjoy a ton of rock running.
There are essentially three entrances to the trail system (Laurentian University behind the sports field, Lake Laurentian Nature Chalet, or from Ida Street) and as it's a loop, can be started at any entrance and can be done in either direction.
The description here starts at the Ida Street Connector and circles the lake in a counterclockwise direction. The trails are very well-marked, and for the Simics trail, follow the red markers. There are also several maps at key intersections, so it's difficult to get lost. The trail can be broken down into two sections, the east and west. The east side is definitely the more difficult with significantly more rock running.
From the Ida Street Connector, head right at the first red marker. You'll quickly encounter a steep, loose rock climb, so make sure you're warmed up and brought a good dose of energy with you. As you head around the east side of the lake, you'll have several short climbs and descents over Canadian Shield rock.
As you pass the Nature Chalet and start heading back towards where you started, you'll still encounter many rocky sections, but the most difficult sections are behind you. Along the west side, there are several opportunities to enjoy the scenery where the trail becomes smoother.