As one former 20 year couch potato to one of current standing, I say to you: you will die (in a figurative sense) if you think you're going to go out on ANY bike a do what you say you want to do.
25 miles? Dead. Hills? Dead. Keep up with your son? Dead. Ask me how I know. OK, not the "son" part but I had the rest pretty well covered

Lower your scope but, by all, means get off your a** and DO IT. You will thank yourself no end.
I know