You can look for routes on sites like RideWithGPS and see the length of rides people do, along with how many feet of climbing there are - a good indicator of how hard or easy a ride is but not the only indicator, since doing a flat 100 mile ride under 6 hours is a good workout, too...

I'm 61 and regularly do 60 - 85 mile rides and an occasional 100 miler. If you were to lengthen your typical rides from 15 miles to 25-30 miles, then you would only need to do three of them, with rest stops in between, and you are doing big mileage.

I used a hybrid bike for many years, did upgrade to a road bike with drop bars and a lighter frame/better wheels - the difference is definitely noticeable from either a fatigue/mile or overall mph at equal fitness view.