I agree with the above. Toronto and JFK are listed at about the same length anyway, but the arrival time is much better from JFK, so I'd do that.

For long flights over 10 hours I stand in the galley for a good portion of the flight as I get antsy sitting too long. During the middle portion when everyone is asleep, the crew is also not walking around much, so there is plenty of room to do so. I also agree with getting a normal day and then going to bed early that first day. If you can avoid a nap that first day it's helpful to get on schedule. Depending on age, a rest day the second day might be highly useful. My parents now need a very very low key day that second day or they are exhausted for a week. Good luck.