There's another trail similar to this on this site, with other picture. I propose two hikes.
The first goes by Mont St Joseph to Mont Victoria, altitude gain of 480m, on a steep climb. The return is fairly gradual. Total for this hike is 13.5kms.
The second hike takes you to the observatory and back by a loop of 15kms. The altitude gain is of 500m.
Getting to Mont M?gantic is fairly easy, from the city of Sherbrooke get on Hwy 212 E until the town of Notre-Dame Des Bois. Turn left and follow the signs to the park.