Keep doing what you're doing, do a couple 35 or 40 mile rides if you want (or just do a 50 miler), make sure you bring enough fuel (food/water), and you'll be fine. If you're worried about climbing, realize that based on the profiles you posted you did almost 3/4 the climbing of the 56 miler in only 1/2 the distance on your 28 mile ride. Just based on the profile, I might even say the 56 miler will be easier for you. You'll be fine. Have fun.