Slow down.  Drink a LOT of fluids.  Eat often (but small) to keep energy flowing and you’ll be fine.  Make some friends so that you can tuck in behind them when/if you get tired and they can do the same to you.  You burn about 30% less energy drafting behind others.  Practice before you get there, though.  

There isn’t much difference once you pass 40 miles.  By that time, you’ve burned off your reserve energy and should have settled into a nice cadence.  The key is to slow down and let your body settle into a nice rhythm for the day, not for your normal 2 hour rides.  That’s why the eating and drinking become so important…for hours 2+.