Hi Robin. When it gets warm, just roll the jacket and tie it around your waist. If your bike has 2 cages, bring 2 bottles. If there's only one, then that should probably be fine (but I'm an over-hydrator, so I ALWAYS bring more than I think I'll need because you never know what's around the corner).
If you know you can do 35, then the 50 should be fine for you. Just take your time and enjoy yourself. Oh- and take pictures because we'd love to see your ride.