When I did my first metric, my usual rides were anywhere between 20-30 miles 4 times a week 18 mph average. Never did over 40 miles before it. It went pretty awful, but that's because the main road of the route was along the beach, with extreme headwinds. Two weeks later did my second one in the city, it went great and ended up with 19mph average. Just stay hydrated and eat along the way. It's really not as hard as it seems. Just stay at a comfortable pace and don't over exert yourself early.