I just did the swim tests about a month ago. I'm close to 51 years old and hadn't really done much swimming since I was in my teens. So, I was very concerned...especially about he 400 meters.
My instructor had me begin with the 800 and then gave me a two minute break prior to the 400. This might have worked to my advantage as it really felt like I never stopped ...and I tend to be much more capable with regard to endurance events (was a distance runner in a "past life"). I was delighted to get a 3 on my 400, 800, and tired diver tow! ha!
What was helpful for me was the following:
1 - Being sure to start slow and gradually work into "my" pace. (Much like running)
2 - Being willing to do the side stroke for a good deal of the 400. While some may criticize this being allowed, I think it's a stroke that is very valuable in rescue scenarios.
3 - I used full foot fins in the pool...also used ankle weight to assist in keeping my fins underwater.
4 - Mental toughness!! I was pretty convinced that it would simply be a mental challenge for me.
5 - Did not bother to use flip-turns, simple found a smooth and comfortable way to more powerfully make the turns.
6 - Took advantage of the "glide" after a turn in order to recover a bit.
Also...I felt like I had something to prove. Wanted to surprise some people! ha!
Happy with 3's...

) ... and good luck to you on your tests!