Novice Swimmer: Training (endurance) Thoughts...

Will try to keep short.... 53 year old geezer in good physical shape. currently completed one of two (stroke lessons) as I've not had a swim lesson in 40plus years. I've been in the pool working out for about 6 weeks now....here's where I am: - have recently treaded water for 30 mins, - can do 2 mins hands above water tread, - swam a mile continuous (little wall push) earlier this week via ***/side/free/back stroke sets (each 50 yards) for 1750 yards. super casual pace and felt good at end - not completely fatigued. - today's swim: 250yds *** 7 mins, rest 2 mins, 250yds side 6 mins, rest 2 mins, 205 yards free 5 mins. I'm not getting muscle fatigue in the 250 free...but finish winded - other strokes I could have gone farther. I continually exhale when face down, but am only good breathing (inhale) on right side...I tried to bilateral breath today - crazy difficult for me. and I'm trying to continuously exhale whenever face down. My goal is to be able to swim continuously for a mile or so (freestyle stroke) without stopping....I think my biggest problem is breathing rhythm(??) Any thoughts/exercises to try? When I practice breathing to left I can't barely do a 50! Thanks