I don’t see how you would incur “strain/injury” by simply increasing mileage. If your bike is set up properly and fits correctly - and your ability to regularly knock off 25-30 mile rides without issue suggests that it is - then the key to extending your range is sensible energy management. If you can routinely do 30 now, you can do 45-50. You should pace yourself, starting out and maintaining a lower speed, taking a break maybe half way, hydrate as you go, and bring/buy some carby snack for the break. Will you be trashed by the end? Most likely, but you’ll be upright, and knowing that you can do it will make heading out theme to time a little less daunting. Do 45-50 enough times that it becomes routine (maybe a dozen rides? YMMV, I’m pulling that number out of the air), and stretching yourself to a metric century won’t seem that crazy. Rinse and repeat until you’ve achieved your desired distance