Swimming is 20% fitness 80% Technique, Anyone can swim like yourself but swimming for any length of times can destroy the even the fittest of people.
By the sounds of it your just pushing through the lengths without any focus apart from pushing through the lengths. You are better off doing 25-50m focusing on technique and resting till your ready to go again. Than just churning out lengths with bad technique.
If your pool has a pull boy (the thing between your legs) use if for 50m then do 25 without, rest then the 25back, do this 3 or times focusing on your breathing and hand entry. This is not about speed that will come very easy once you have cracked the hand entry and breathing. Get a book called swimsmooth loads of them second hand for next to nothing on the forest, or book 5 lessons better still join a club, the coaches will spot everything wrong very easily within the first session then its just down to you sucking up all there thoughts like a sponge. Personally i love it when a coach picks something out i am doing wrong or improve on. Videos and books work very little with me i am afraid. So slow down and keep at it, focus on technique not distance.
(do not be fooled by my time really, swimming in a wetsuit is superman compared to a pool)
Tried the pull buoy, what a difference, explains a lot, basically my legs are the problem, must be creating too much drag although I actually thought they were an asset.
With buoy I'm a proper swimmer, I can even see what I'm doing with my hands.
So how do I go from buoy to no buoy yet keep the legs and hips up?