Four training days a week is plenty and splitting your work days up is the most sensible way to do it. Even though you have a physical job you can recover on your work days,especially if you make sure you eat well during your work day.
If you are dealing with age and a “run injury” I would gear training to a swim/bike focus with your run more of a run strength program than “a marathon” program. I wouldn’t even worry about doing long runs every week,I’d do them every two weeks as you have stated that you plan to walk/run the marathon.
I see your biggest issue in your swim training access .I would move heaven and earth to search out a pool somewhere closer than the huge commute you currently face,even if it means joinig a gym with a pool.
At your age,sleep and recovery is important so make sure you get plenty and again,at your age some resistance work would be a good idea ( I am 60 ). First timers tend to freak out and worry too much about race day early on in training. Forget about the race and worry about your training blocks. I always say “train to get strong enough to deal with the training you will face in two months and repeat,and repeat util the race is much,much closer” . At your level overall fitness is what matters to start with. You are already used to long,physical days at work. Ironman should be easy by comparison.Ya, the swim part can be difficult. We do have a public pool here in my small town, but it’s only open during the summer. Those summer days I try to get at least two days a week training in the pool. The rest of the year, I will drive to Lake Mead 2-3 times a month (about 200 miles round trip) for some open water training. I would not call myself a fast swimmer, but I definitely believe myself to be a strong swimmer. By that I just mean I’m very comfortable in the water. No panic even in rough or crowded water and can maintain my modest pace for a couple hours.
And the break up of my training block is exactly why I’m thinking of this. A 12 hour shift can be pretty draining, but physically, it’s not as demanding as a hard training block. So I’m thinking work two, off one, work one, off three. That would give me a one day break in my week to get a harder training session in. As it is now, with four off, my first three are more moderate training sessions and that fourth day, right before I go back to work, is a high intensity or long duration day. (of course, moderate is all relative now. What I call moderate today would have killed me 2 or 3 years ago. I’m sure everybody can relate to that.)
As for the run. I really used to like running. Back in my Marine Corps days, I used to run just for fun. At one point, I was running 75+ miles a week, besides the organized company exercise. But unfortunately, I’m on the smaller side anyway (well, overall. Maybe not so much for a woman) and carrying the weight we sometimes had to carry takes a toll. Didn’t help that I was too stubborn to ever drop out of marches. Always in the back of the pack, but never in the truck.