The distance sounds daunting in and of itself, but it is potentially doable.
What concerns me is the road quality (the pavement around here is just terrible) and the number of hills that you'll be facing... I live in Sherbrooke and haven't thought about making the (much shorter) trip to Montréal by bike because I am not sure that I could handle the brutal climbs.
What's the longest ride you've been on to date? I ride for 3 and 4 hours pretty regularly, but just completed my first century (164 km) about a week ago - it took almost 7.5 hours, and I found myself pretty saddle sore after the fact (I actually had bruises).
My longest ride before the century was about 68 miles (109 km) and I ride daily, too - but I was told that I should have worked up to the longer distances more gradually to make sure I didn't overdo it and injure myself.
I'm admittedly new to long distance cycling, but my best advice is to be sure that your saddle is going to be comfortable for a trip that long, have comfortable padded shorts (gel was recommended to me), and bring along some chamois cream or bag balm for any rides that long.
I can't give you much advice on nutrition and hydration for a ride that long because I've never tried it, but I found it difficult to carry enough water and gatorade just to complete the century, even with a saddle bag.
Are you planning to ride back to QC after your visit, or take the bus back? That's an awful lot of riding if you are unaccustomed to that kind of distance.