I love this park! It's a great place if you're looking for a good workout (power walk, run, cycle) or if you just want to go for a leisurely walk and feel like you're outside the city. If you head towards le chalet du Mont-Royal, then you will be able to tke advantage of their great lookout, which offers beautiful views of the city. There is also another fantastic lookout (the view is much broader, I think) but I've only ever been there by car, so I'm not sure if it's accessible on foot. The length of your workout or walk will depend on where you start - there are a few entry points, including the base of the mountain (I forget if that's on Berri...?) and via Duluth, Peel etc.
Suggestion: Keep an eye out for cyclists to avoid being run over :)