Speed

I can't seem to improve my speed. I basically taught myself to swim as an adult, with a few tips here and there from lifeguards who would see me swimming. My stroke got decent enough that I could swim 9 miles in open water. But my speed was and is bad...I swim a 33-35 minute mile, both in the pool and in open water. So this summer, I went to coach here in San Diego who has coached a lot of channel swimmers and he totally changed my stroke over the course of a few sessions. He said I should definitely be able to swim a sub-30 minute mile. Now, I like the new stroke a lot but it hasn't really increased my speed! What is going on? My coach suggested that I breathe every two strokes because he said oxygen is a limitation, but I kind of rejected that--i feel a lot more balanced if i breathe on both sides. Should I listen to him if i want to get faster? If my stroke is good, does that mean i am just slow because i'm not working hard enough? Should I go for more sessions with the coach? I'm not trying to win races or anything, but it just seems odd that the old guys more than twice my age are swimming faster than me!