Don't know how fit, or unfit you are, so hard to say. Some folks are up to that distance in a couple of weeks, others have to work up to it over weeks, or months.

25 to 30 miles is a very doable distance. It is only 2 hours in the saddle at a slow to moderate pace. Even if you start off very unfit, I would imagine you could work up to it within a few weeks to at most a couple of months if you ride, say, every other day for an hour to an hour and a half per ride. When I was your age, I remember riding that distance seemed like a challenge, and I only did that distance maybe once or twice a year. Not any more. though I am 48, almost 49, and somewhat heavier than OP, 25 to 30 miles is a kind of benchmark I try to hit early in the season, then go from there. This summer, I have been doing more back to back rides of 30 miles or more. I hope by the end of the summer to do a kind of "tour" of 4 to maybe 6 days consecutive of 40 to 50 miles per day. Don't know if I will get there this season, but it is something I am shooting for.

As to what other exercises compliment cycling, I would say just about anything. Weight lifting, Pilates, yoga all come to mind as non aerobic activities that will work your body in ways cycling doesn't. Running, swimming, rowing, eliptical machine, and spinning (for bad weather days or for working on something like climbing or high cadence) are all beneficial and complimentary aerobic activities.