swim faster in 17 days? sprint

If you're swimming a half mile in 24 minutes, and given your big lack of swim-specific background/training, it is almost certain that you have not just a technique limiter, but a HUGE swim-fitness limiter. 

Remember, just because you are a strong runner/cyclist, does not mean you can jump in the pool and expect to be fast right away (unless you happen to be extremely talented at swimming.) In fact, you will probably be as good as any ol' nonathlete learning swimmer in the pool in the beginning, which is about a  24 minute half mile pace. 

In order to tap into your aerobic engine you built running/cycling, you must have the muscular endurance in the swim-arm motion to match. This takes a lot of training and hard work. 

It is a total myth that people should be swimming at even MOP speeds with zero swim fitness, and just 'magic technique.' You need swim-specific muscular endurance, and running and cycling do NOT build this whatsoever. On the bright side, you should improve pretty quickly with your ability to suffer based on your run/cycling training.

And yes, technique technique technique is still the name of the game until you're readily swimming 2:00/100yds paces for distance - you'll pick up the low-level musular endurance you need at that level just from technique training. After that though, gotta start cranking it up.

I think this applies more at longer distances.  I swam and played water polo in school.  Now, many years later, I could jump in and swim a fast 100 without  much in the way of workouts.  But the pace dropoff at 500, or a mile or more is terrible.  And keep in mind most triathlons are at least 800.

Even more importantly, if you did ANY significant water sports, be it club swimming, YMCA swimming, or water polo, in the past, yes, of course, you can either come back to speed MUCH faster or even be pretty decent right out of the bat even 'out of shape.'

This totally does not apply to the OP who has zero swim background and has never swam faster than 24min for a half mile, he's got to put in some work before getting up to speed. 

It's no different with running and other sports - if you had it before, you can get it back much, much faster than someone who's getting it de novo. This doesn't mean that your swim technique is solely responsible - recovering fitness is an enormous component - just look at the arm turnover of someone going 1:30/100 after a long layoff compared to 2:20 - you cannot technique your way to that sort of arm turnover. 

If swimming was almost all technique for speed, and very little fitness component, people would be getting very fast by just doing slow drills and refining strokes carefully. But watch how any good swim program works, and they swim hard, and a lot. 

While you're at it, tell your GF that swimmers don't "scoop water"......they pull themselves through it.  You aren't moving water, you're moving your body.  BIG difference.