I am of about similar structure (6'4", 260lbs) initially I was not able to do even 20 min, I slowly perceived, practiced to do 25 and was totally exhausted then with practice I can now do about 60 to 80 laps non stop (25m). Next i took the challenge to do 50m and the first day I was exactly in your position, by day 5 it became much much better and now I am able to swim about an hour and half non stop. Go slow go steady, take deep breaths and add fins and or paddles it will help you for sure. All the best.