Way to go, Ash... I did my first 100km in December last year (about 62miles)

The general advice in training books and articles, is to increase your riding by about 10% each week.
So do take Lisa's advice and try a couple of 25-35milers before you set out for your 50 odd miles.
We are asking our bodies to change and respond at a metabolic level, so we need to ease into such distances carefully.

Having said that, just set yourself a gentle pace, and you will be fine!
Looking forward to hearing about it