Flybye, here is a sample 10 week training plan for a 100 mile century. Jo listed the longest ride for each of those weeks. Take the chart from this article, do some math and scale back for a metric (62 miles). You can amend the schedule down to 3 rides minimum a week. 4 is better.

http://www.kanbike.org/pages/training.php

The LRRH route is very flat but the home stretch can be a headwind all the way home. You need to spend time sitting on that bike. A good rule of thumb when training for such events is: train your legs if it's a hilly ride, train your butt if it's a flat ride. You need to spend time just sitting on that bike going for longer and longer rides.

You have plenty of time between now and then. On event days, your body will find a magical source of energy and take you farther than you expect, but the more training you can do, the more fun you will have on the ride. Be sure to taper the week before the LRRH.