In my experience with exertion at altitude (backpacking, not cycling, in New Mexico, between 8000 and 11000 feet), allowing a day or so to acclimate to the elevation was sufficient. I certainly noticed that I was not as sprightly as at my normal 200 feet above sea level, but it wasn't debilitating, even climbing hills with a 40-lb. pack.

If your corner of Ontario is so flat that it's hard to train for climbing, that might be more of a problem. If you're near the Niagara Escarpment, could you go do some climbs up it for practice? Alternately, you could do some training riding into the wind on a relatively high gear (big chainring and a small-to-medium rear cog), to simulate the exertion required to climb hills.