I think they have something similar here in Britain, only in meters. I've never tried doing it. Perhaps if I could get a similarly slow swimmer to accompany me it might be interesting. At the moment I think I could do it with frequent rests. I suppose the plan would be first to do five minutes then ten and so on. I may swim an 800 meters soon, which would give me an idea of a suitable possibly sustainable pace.
I must have a look at the USMS site and see what the over 80's are capable of there. Some of them are seriously fast and have serious endurance. The trend here seems to be that the older swimmers gradually shorten the maximum distance they are prepared to swim.
I understand the policy is to start out much slower than what you think is the right pace.