Werner,
It sounds like you have found your 'jogging' pace but not your walking pace which is why you can managed a long swim (45 minutes) but not an all day swim.
I think the answer is in balance comfort.
I am similar to you, I think I can currently do 75 minutes but definitely not 4 hours.
I guess once we learn to stop using energy on maintaining a float position then the easy walking swimming comes too?
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