If I recall, it is about 20-25 miles from Petit Lake to the boat dock at the far end of Redfish (Tack on about 6 more miles if you hike clear to the lodge).
We have done it, but has been many years and we took the trail to Edna lake and then to Cramer lakes. From Crame Lakes to Redfish is pretty much all down hill. If I remember, it is a very scenic hike that isn't too difficult.
Should be a good trip for 3-4 days of leisure travel.