This thread sure took a turn to something totally different. I thought OP just want to get back in some sort of shape after artificial knee and all the other aging problems!!
To be honest, at our age, just be thankful we still can punch and kick. People might be very good when they were young, but it's a PASS TENSE!!! You get old, you slow down. The key is trying not to become a vegetable. You are NOT going to be as fast as some young people even they have less training.
To OP, just start working out. Believe me, it's going to be a long road for you to get back in shape. Forget what style that is more useful, you learn so many different style. According to you, you were an expert back in the days. You already learn Jujitsu and all that, adding one more is not likely to make you a better fighter when comes to bare knuckle. Why don't you just start training on things you know first, get back to some sort of shape. Believe me, it will be harder than you expect. Then if you really worry about self defense and not carrying a gun, learn stick fight like what I've been doing in the last 6 months. That will give you a clear advantage over any unarmed people even they know MA.
Like I said before, I never stop doing bag work, for 30 years, I put in like two section a week, about 15 to 20 minutes each section as aerobic exercise. I put more time in weight training. To put it in perspective, I lost count on how many EverLast 70lbs canvas bags I broke by punching and kicking, I am on my 4th 100lbs EverLast heavy bag, two canvas, then the last two were leather. leather last longer. But now, both my bags are covered with duck tape already. So I am NOT exactly playing pussy foot with punching and kicking.
Around 2 months ago, I had my wife recorded me doing punching and kicking, IT WAS A BRUTAL AWAKENING!! How sloppy I was, how out of everything I was!!! I really took it to heart and start doing it with more heart and really work on the technique that I had and lost. Together with the stick fight, I can tell you IT IS HARD ON MY BODY, I NEVER EXPECT THAT. All the aches and pain, the joints start complaining all over. My back was screaming, my elbows, forearms and even my neck were in pain. Took me like 4 months ( from starting of stick fight) to calm everything down. I am still fighting the left elbow pain.
You have NOT been working out for so long, I think you are going to have even a harder time than me as I was still working out like 4 hours a week before the brutal awakening!!! I was never out of shape, Just adding more hours and it's a day and night difference on my body already.
Be patient, start slow, don't give up, DO NOT EXPECT too much, JUST DO IT. forget about kicking butt, just make sure you can still do what you had before. Then if you are up to it then, think about learning new stuffs. Your body will let you know, take my word on this one.