Unfortunately, it is a myth that you can "strengthen" your lungs. Your lungs are a passive gas exchanger built like a sponge and since your COPD has "clogged up" a significant part of the sponge your lung capacity is dimished. And age alone causes degeneration of lung capacity which is a major limiter in the athletic performance of older athletes.
I am 52 and have asthma so I can relate. I use advair daily. I use an albuterol inhaler 15 minutes before I swim. The COPD can limit the effectiveness of bronchodilators but you might want to check with your doctor whether there is more medical management that could be done. Pursed lip breathing can also help.
I find that my asthma really limits my ability to go "all out" for an extended period of time because if you breathe too hard you really limit your air exchange. I find I have to be very conscious of keeping my breathing under control.
Where TI can really help you: TI proposes that better swimming is mostly achieved by better technique, proper streamlining, reducing wasted energy, neural training, and much less by calisthenic overtraining.
I think you will find that you get great improvements by focusing on technique and pacing -- take it slow and easy and train yourself to breathe with ease.
One problem that often comes up is that TI promotes a longer stroke length and lower stroke rate. This means you breathe less often. You can deal with this in several ways -- by taking frequent rest intervals, or by rotating to sweet spot and getting several breaths before continuing, or by keeping your stroke rate faster but compensate by pulling less vigorously.
Try to keep your heartrate fairly low, that can keep you from overtaxing your lung capacity. For longer distances I try to keep it under 130 or even lower.