In recent years, the most compelling scientific evidence for diabetes management has been the inclusion of resistance/strength training as part of an exercise routine. Why is it so important? Think of it this way: our muscles are the main place we have to store any excess carbohydrates that we eat, and the bigger the muscle "gas tank,” the more carbs we can store there.
The sad reality is that getting older by itself causes some loss of muscle mass over time. If you go on a diet to lose weight, you'll also very likely to lose some muscle and not just fat--unless you resistance train. Being sedentary and having diabetes both increase the rate at which you lose muscle, and older adults with diabetes frequently have far less muscle than other people the same age. Only strength training recruits and preserves the muscle fibers that you would otherwise lose as you age, sit around too much, or have out-of-control blood glucose levels.
Being inactive only makes the problem worse because not only are you losing more muscle faster by not using it (think muscle atrophy), but the "gas tank" remains full. You're more resistant to insulin (whether it's naturally release, pumped, or injected) when your muscle carb stores (glycogen) are packed to their limit, which happens when you eat a lot of carbs and remain inactive. Any carbs that are unable to go into storage in muscle (or the liver) raise your blood glucose and then later are converted into and stored as fat.
You don't even have to join a gym to make this happen. Simply start doing strength training at least two to three days per week by doing exercises using your own body weight as resistance (like planks, lunges, or wall push-ups). Resistance bands, dumbbells, and household items that can be used as resistance (e.g., water bottles and soup cans) will also all work.