Trying too Hard

I have read so many training plans that I have just ended up confused.  They seem to contradict each other.  Anyway, today I did a short run on the treadmill then went for a swim.  Struggled to swim a full length in one go, I was just exhausted.  I managed 6 incomplete lengths then gave up.  I think I am trying to do too much too soon.  So I will rest tomorrow, then start again, but just do one discipline a day.  I have found a training plan which works on that principle, its for beginners too, so I will try and give it a go.