Thanks for the advice guys. I think the reason my 70km ride was a breeze is because I have figured out that I need to stop more often for short breaks. Before I was trying to see how long I could go before needing a break, but now that I take 5 - 10 minutes every hour or so the ride is much more manageable. I'm not concerned about speed and if I have to take 20 breaks on the 200km, so be it.

I plan to leave Ottawa in the morning, not too early because I am not a morning person and I have most of my energy in the evening. It will be dark when I reach Montreal, but because I am riding toward home I will be more comfortable in familiar surroundings in the dark.

I think I will do as Machka suggested. Work up to 100km, do that a few times, then a 120km, rest and then go for the 200km. My only problem is I'm going on vacation to Spain for 2 weeks in September and will somehow have to work in a long ride on a rental bike while there. That'll be interesting to say the least!