Swimming is harder than it looks

So yesterday was cross training day and I thought I'd take advantage of that and get a little more mileage on the vibrams to build up the muscles slowly without interrupting my half marathon training runs.  I put on about 3/4 mile and felt pretty good.  I felt a little more pounding yesterday than on Monday but overall it was good.  Then I went swimming for 30 mins.  I must admit I've always been able to swim but I'm by no means a good swimmer and so lap swimming is very difficult and quite the workout right now.  I assume I'll get better with time but I did about 15 laps in 30 mins and was exhausted.  Scale is down a little but not much.  I really need to look at what I'm eating, might do a log for a couple days because I'm just not dropping the weight I would expect for the effort I'm putting in at the gym.  Today is an easy 3 miler, might go 3.5 just to get ready for the long run this weekend.