After a long lay off Master Swimmers should take it EASY!!
If you have been out of the water for over two months, your first work outs should be slow and easy trying to hold good form.
A good start would be to do 1 mile or 2000 mtrs a day. Stop as many times as you wish until you can complete the swim without stopping. Don’t increase your distance during the first week. After the first week, and once you attain the nonstop swim, go an extra 500 or 1000 mtrs. of easy swimming.
Compare yourself to a race horse that has been turned out and rested. Go at a leisurely speed or you will breakdown. This slow work builds the capillary system.
A race horse jogs or does easy work for six weeks before it starts to do timed work outs. Don’t do repeats or sprints until you are ready. Be sure you take one or two days rest each week.