Little fishes.... How come they arn't knackered half the time ?

I sneaked in a quick 20 mins swim today as needed a break from work. Good session but blow moi, as I tried to alternate strokes from breast and front crawl, I was totally puffed after the front crawl so how on earth do I get over that ? The swimming coach was there and we had a chat after. She gave me some tips to do with breathing exercises so keen to try that next week. In the end, got a ski trip coming up so need to get a bit of practice at  the local slope on Sunday and still no bloody bike. I guess  that's my  April focus ! Bring on that birthday....... And he bike as my present.....