Very out of breath swimming
Any suggestions or theories welcome. I'm learning front crawl from scratch and have been having lessons for about 8 weeks or more. I now have the basic technique but I can't do a length without being very very out of breath and this doesn't seem right based on my general fitness (eg: I can run 5km around the 20 minute mark fine and can cycle a 20km in a Sprint tri around the 35 minute mark).
My coach says I might not be expelling enough air but other than that doesn't seem to be able to help. I'm breathing on every third stroke, left then right. Am I kicking my legs too fast? I've been told to kick fast to maintain buoyancy but it seems to be a bit like sprinting underwater but only breathing on every third stride. Everyone I see in the pool seems to be going as fast as me but able to maintain it indefinitely, whereas I'm stopping on each length - and of course I know that's not right.
Do I just need to shut up and get on with practising or is there anything in particular to watch out for??