I did my first metric last summer after riding less than a year, and after I'd only done a couple of rides in the 40-45mi range after being off the bike for a month. It was super hilly and hot, and I was miserable (riding with a fast climber), but I survived. I have no doubt that you can do it. Just keep ahead of the bonk. Eat in the neighborhood of 300 calories/hr and keep the hydration and electrolytes up to par. I can do that same ride no problem now (and hot but flat and fast 70 milers too). Only my butt starts to complain by the end. You've got support and can go your own pace. Sounds perfect.