Call it 'prehabilitation' since it seems that having a catchy new word for your idea is important in doing health and fitness writing. This is an idea of mine, about something to do before or concurrently with the start of your aerobic or weight-bearing exercise program. The idea is, most of us live our regular life without exercising our bodies much. Especially, without challenging them much. In my routine work day, for instance, the heaviest thing I life is a book or maybe 2 books. Desk jobs are good for that reason, and they're bad for that reason.
If you don't use muscles, they get weaker. If you're several years past the last time you exercised regularly (and not just some sort of exercise, but exercise that challenged your arms, and legs, and abdominals, and ... on down the list), then I think you are liable to have some areas that are not currently in good shape. What that means, as I've coached beginning runners, is that you're liable to start your exercise program and have some real complaints from your body about it. You can ignore real complaints, for a while. The 'while' is however long it takes your body to make an injury out of the complaint. The easy conclusion -- that you're "just not cut out to run" (or swim, or walk, or ...) -- is the wrong one. The better conclusion is that your shoulders were used to not having to work, so when you made them do so in the pool, for the first time in 15 years, they complained. The swimming itself is not a problem, but having let your arms get used to the idea that they only needed to lift a book or two every so often, and then ask them to pull you through the water, was the problem. Same sort of thing for many other sorts of pains beginners encounter. (There are also many that have nothing to do with this. This is, alas, not a cure-all, even if it does work at what it tries to.)
Hence, prehabilitation. I have had to do rehabilitation, at one time or another in my life, on my knees, shoulders, and back. These seem also to be three major areas where people encounter problems. Since they are such common areas for problems, I suggest that doing rehabilitation style exercises is a good idea -- even before you're injured. Especially before you're injured.
As always, check any exercise advice out with your doctor, or, at the very least, a certified trainer in the sort of exercise you are trying to do. I'm serious about this warning.
This is an idea of mine about exercise -- not about sea ice. If it's about ice, there's a fair chance that I'm right, even if I'm disagreeing with a sea ice expert. On exercise, well, I've tried to read a lot and listen to people's experiences. So I've got a fair number of ideas, and they're probably not too bad. Usually. But I'm not an expert here. The fact that I've never seen this suggestion in any of the books I've read, or heard it from another coach or trainer concerns me that the reason is that it isn't actually a good idea, rather than I happened to have been creative and thought of a good new idea.
In terms of which exercises to do, I can't provide much guidance myself. So, again, check with your doctor, letting them know that this is the line of thought. For shoulders, for instance, one of the exercises is to take a weight (start with 1 lb, a can of veggies works fine for this purpose) in your hand and arm straight down at your side. Lift your arm straight out to the side, up to the point that your hand is shoulder level -- no higher. I've you're recovering from a torn rotator cuff, as I was, this is challenging. Complete the set of 10-15. If you haven't torn your rotator cuff, and this is hard -- you need some serious prehabilitation. Work to the point of being comfortable with 10-15 lbs (5-7 kg). It'll take time. And, note, this is not the only exercise you should do for your shoulders. There are several more, just of this sort.
A simpler exercise that anyone and everyone can do, pretty much anytime and anywhere (ok, might look a little funny in the wedding party), is to stand on one foot. Just that -- stand on one foot. If you're not positive about your balance, be near something stable you can grab on to if you need to stop from falling over. You want to be able to do this for 60 seconds, easily, without having to grab anything. When I started it up again most recently, I could make about 15. Work on both legs. One is probably much more stable than the other. But you want to be getting 60 seconds with each. Once you've made progress on that, try it with your eyes closed. (I get 3-5 seconds this way at the moment.) What you're working on here is your awareness of where your body is, and what your balance is. Apparently, since I did it, you can be really lousy at this and still walk/run/swim/.... But it's easier to do those things if you have better balance.
A different reason I like this idea is a problem many beginning runners have. They're fired up and want to go run every day, now that they've committed to doing exercise. And then I (and most coaches, especially anyone certified by the RRCA) tell them to start with running only every other day. There goes some momentum on making their change.
So I'll suggest that the thing to do on your non-running day is some other sort of good exercise. Maybe prehabilitation is a good idea for you, maybe it's the weight room, maybe yoga, .... There are many options here. But in terms of setting your schedule and getting habitual about your exercise, there is a big plus to having a particular exercise time. And then you decide which exercise it is that you're doing for that day.
I'll invite contributions of sites that give good prehabilitation-oriented exercises. We're not aiming at the 'how to put on 30 pounds of muscle in 30 days' or 'how to lose 30 pounds in 30 days', but some basic, conservative exercises that people with injured shoulders/knees/back/... could benefit from, and that someone who isn't injured could also do and benefit from.