One rule of thumb is, miles per week equals long one-day ride. So your almost had the weekly miles for a century.
If you're starting with "just riding around", any kind of intentional training will help - intervals, tempo, spinning more, etc. Fun is good. I enjoy intervals up short hills, so i do that.
Rest days are equally important, and increasingly so with age. The harder you train, the more restful your rest days need to be
On a century, hydration and nutrition matter more than even 60 miles. Don't fall behind early on eating.
And there's always this: ride lots.