How to gain swim strength?

They swim pass by you because they have better swimming technique and overall they are better swimmers and not because they have more strength. Do you really think a 12 y/o is stronger than you? Anyway, in your case I wouldn’t worry about strength, I would just spend more time in the pool working in swim technique, form and feel. Plus, spending more time in the water will supply you with plenty of strength training...

I struggled with swimming at first.  I would shoot to swim for a certain distance, and I would get that distance by hook or crook -- using every stroke imaginable, and some that aren't...

I realized I needed help.  And I started doing the 3 month beginner swim plan here on BT.  It had 3 workouts/week.  One drill wo, one endurance wo, and one combination.  It was amazing how much the drills helped.  My total distance on workouts now goes to 1400m.  I can live with my time, and I feel comfortable in the water.  I am by no means done but I like where I am headed.  

Doing drills and endurance training over four weeks will help some... but mainly it will get you in the pool building base and things may snap more quickly for you.

 Peace,
Brian

I started swimming this past February.  I couldn't perform the front crawl to save my life.  I used the Total Immersion techniques and basically taught myself the technique, which I am still practicing even today.  However, five months ago I couldn't even make it to the end of a 25 yard pool without having to stop and catch my breath.  After swimming for at least 3 days a week for the past couple of months I am finding my endurance to finally increase.  So to echo what others have said, swim, swim and swim some more.  Practice, Practice, Practice. 

Good luck.

One other thing that will help, is when you are swimming, at least now, don't worry so much about your speed.  That will come, particularly as your form increases.  I've found that my speed has improved greatly from taking Dori's advice from Finding Nemo " Keep on Swimming, Keep on Swimming" 

Also, at least for me anyway, Focus hard on one portion of your stroke at a time.  Trying to fix improve all of them simultaneously is difficult. You need to concentrate on the item that you are working on.  For me, it was the "zip" of the stroke first, followed by bilateral breathing.  Then I started with the kick... etc, etc.  I'm far from the best swimmer in the pond, but I'm improving with each workout.  It DOES take time!  GOOD LUCK

The three most important things to think about if you want to improve your swimming speed:

 Technique, technique ... and technique. 

Oh, and swim lots! (I know ... I am sort of quoting Merckx here ... but I just had to!)

the zip drill is performed when you swim freestyle and as your arm returns to the top of the stroke (ie back up to your head and out in front of you) you deliberately zip the water up - ie drag your fingers across the top of the water.

the benefit of this is to force your elbow high thus improving your stroke entry.