A 5 minute mile past 40 isn't a huge ask if you had the speed to do it at one point. I would have guessed that you would have had about 4:45 in capability in high-school when you were relatively untrained, so at least 4:30-35 trained, if not quicker. At 40, I was quicker at a mile than I was at age 17, having raced throughout the intervening years.
I lost some time with age, but I think I ran about 4:42 at 46 (not much slower than what I ran at 17/18), 4:59 on road at 49, and just on 5:00 equivalent for 1500m on track at 53 (only two days after a 5000m).
I think you have time to play with - both in your basic level of talent and in a few years before serious age decline sets in. I would make it a two year, or even two-and-a-half year plan, which gives you a chance to get there without injury.
You have to keep in mind that you're "engine" - the cardio-vascular systems - will develop quicker than the "chassis" the muscular-skeletal structure, which is where the danger lies.
I think you could get fit more quickly with intensive training, but there is a good chance on getting injured before you ever get where you want to be. I would spend most of the next 12 months developing "the running body" and just gradually building some aerobic background, and just throwing some easy strides into runs to wake up the fast twitch. It might sound crazy, but even something like a 12-week "couch to 5k" program might be a good start for someone who hasn't had a running background. Running the odd 5k on a program like that would give you some competitive fun on the way.
I've no doubt that you have the natural ability - genetics, cardio, biomechanics - to run well under 5:00. I think you've got enough buffer that it's not even going to kill you to do it, everything is going to depend on getting steadily fit enough without long-term injury.
Good luck!