Exploring levels of relaxation

I've been getting tired of late when swimming longer sets - just feel like I've been using too much energy. So with half a mind on the Summer ahead and some Open Water swims of 2K to 4K and possibly (if I can talk myself into it) the Lake Coniston swim of 10K in September.

I've been swimming at survival super easy pace a bit for warm ups of 1K -  letting the water take the weight of my head, catching and holding but not pulling and yet still weight shifting. So relaxing - if you don't often swim like that I recommend it.  

Today I did nice tempo session:  3 x 400 at 1.20, 1.16, 1.12 then 4 x 200 at 1.08, 1.06, 1.04, 1.02.  Then 1 x 200 at 1.06 (1.06 felt super easy).

Yes upper end of my Green Zone, but super comfortable and still focussing on the same things. Relax and let head be fully supported, weight shift, catch and hold with featherlight pressure.
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Relaxation is the key.  How many times do I learn that lesson?