I'm 40, and I've been involved in studying middle-aged fitness for some time. Things to know:

• Most research on fitness is conducted on high-school, NCAA, or oly/pro athletes, or military populations. In every case, we're talking about populations that are overwhelmingly under the age of 25. Bodies that can take more punishment, that recover faster, and that don't show the consequences of over-training injuries until years/decades later. Don't just blindly follow the traditional athletics ideas as facts - they may not apply to you anymore.
• You cannot push a body to perform better, be healthier, or so on, not if it is already in generally good shape. You can finesse improved capacity, but that is done by carefully managing workload and recovery, and can often come with tradeoffs.

Lack of mobility:

• For ankles and knees, I strongly recommend the ATG split squat with the back leg kept as straight as you can manage. You don't need to load it, it's just a basic load for the knee and ankle at full flexion. You can put your front foot up on a raised platform like a chair or stair if they feel hazardous off the ground.
• There is only one resistance exercise that peer-reviewed science shows prevents sports injuries: the nordic curl. It's a movement that pulls the knee joint closed in a better position for knee health than a leg curl, used to eliminate imbalances in the knee caused by typical overtraining of the opening of the knee (e.g. squats, rucking, running, jumping).
• Stretching is huge, especially as we get older. Since older folks can benefit from longer rests between sets when lifting/resistance exercises, what I do is stretch muscles I'm not working between lifting sets. Between squat sets, I'll stretch my upper body; between upper body sets, I'll stretch my legs. It lets me get more time into my workouts, but also doesn't eat up more time in my day since I should be dedicating time to stretching anyway.
• It's tempting to do lots of upright exercise, but getting low on a routine basis is huge for lower body mobility. Find a yoga or martial art form/kata/jinga that gets you into deep knee bends; deep squat and hold it so your knees/ankles stretch a few seconds every time you walk out of a bathroom, or have your watch chime on the hour and just do it then. Make it less of a scheduled workout and more of a constant part of your existence to get low and shift weight around. If you make being mobile a thing you do a couple times a week and need to "recover" from, you'll never really get comfortably mobile.
• Given your back injury, you need to get a gameplan from a PT that is familiar with your medical history specifically. It's the best money you can spend if you haven't done it already.

Illness. I have a family, none of us have illnesses requiring long term meds but that terrifies me. Like in One Second After the daughter dying from being a diabetic. Add that to older age stuff like dementia/Alzheimers how do I mitigate this?

My wife is on substance-controlled medications; her life as we know it ends the second her meds run out. Unfortunately, there isn't that much that can be done beyond keeping a supply, having a backup plan to procure more from another country, and then accepting fate and focusing on more productive living and prepping. For old-age mental stuff, you can mitigate it to an extent with a healthy diet, exercise, and staying mentally engaged with learning. Good learning like textbooks and quizzes and book reports, not just face-rolling through Duo Lingo to keep your streak alive. It's also good to have a plan regarding what happens if people start to slip.

Can't control it all; don't waste your life worrying about it.

What fitness things do you all focus on?

The general consensus in the medical community is 150-300 minutes of aerobic exercise per week, and 2+ resistance training sessions. For older folks (40+), I do and recommend the following:

• Rucking with no more than 25% of your body weight, no more than three times per week, and no faster than a brisk walking pace. This is not a good place to push yourself on weight (can fuck your joints) or speed (can extra fuck your joints) because rucking can be such a good low-impact low-intensity-steady-state (LISS) workout if it's not overly strenuous. The healthy challenge would be to try and ruck for a longer period of time in a single session, up to a couple hours. Rucking is, by far, the best cardio around if done safely.
• Lifting weights on a split designed to give at least two rest days between workouts. A 3-day PPL split is great, or a six-day PPLPPL if you'd like - just be careful regarding shoulder overtraining on your pull days when they already get worked on push days. Another split scheme that is good for health/maintenance but is hated in the strength/hypertrophy community is the bro split: M-F each having a separate muscle group to work (chest/back/shoulders/arms/legs). It's sub-optimal for muscle development, but everything get a week off so you can go as hard as you want without risk of overtraining.
• I don't recommend full-body workouts. It stacks too much fatigue into a single session, limiting your ability to go hard in the last half. The goal of resistance training isn't to feel exhausted but to work muscle groups - pushing for whatever feels the hardest is not a good analog for whatever will develop the most work capacity, strength, or size.
• 2-4 minutes of rest between sets in your 40s, and up to 5 minutes of rest in your 50s and beyond. The body doesn't recover anaerobically in old age as well as it did in our youth, so longer rest periods means better performance when lifting. Again, lifting is not the place to train aerobic capacity for exercise - that's what the rucking is for. It's tempting to to push to exhaust everything all at once, aerobically and anaerobically, but the reality is that's a lot like spinning wheels - putting mileage on the machine for zero benefit but to feel cool in that moment.
• Like I said above, I stretch during these rest periods. For me, it ends up being about 2 hours of stretching per week - massive in the grand scheme.
• Incorporate some form of plyometric (explosive) exercise once per week. This can be box jumps, sprints on foot or bike, tire flipping, sled pulling/pushing, or hitting a heavy bag - it doesn't matter. Manage it carefully and don't go looking for ways to increase performance much, because it's an easy place for catastrophic injury. But plyometric exercises have no substitutes, so the first time you need to jump high or hit hard or run fast after not having done those things for years is where you can get into quick trouble real fast. I like to rotate: box jumps and punching bag one week, sprints and tire flipping the next, spin bike HIIT and sleds the next. That way, anything that gets tweaked has a good chance of getting a lot of rest before going again.
• Spin bike is a magic recovery machine for the lower body. Not the same as a bicycle. It's truly zero impact, and with low resistance it's just a method of pumping blood into areas that need blood to recover. I'd wager the only thing better is your own swimming pool. They can be had for cheap on Amazon, and I use mine on all of my "rest" days.
• Rest. 8+ hours of sleep, managing recovery, being willing to turn scheduled workouts into rehab sessions if that's what your body needs. All the spreadsheets in the world can't tell you how your body is feeling right now, and it needs to be the driver in how you decide to exercise.