I'm mid 60s also on Lantus & Humalog (and anticoagulant) - if I'm swimming (most days) I reduce Lantus dose on day before by 1 unit - same with pre-swim Humalog. Eat approx 2 hours before swim, aiming to have BG around 7 at start of swim.
During swim I drink carb drink ('high five') which has 44 carbs. Normally swim 3000 to 4000metres. BG usually drops couple of hours after swim but will have meal to increase.
If swim is intensive, BG can be quite low post exercise - I test immediately I'm finished. I carry 'high five' gel in case it is too low (which I also use on longer bike rides).