Could i improve much at middle distance? Im 26...?

You are still young. LOL. I am a distance runner and I can offer helpful advice. I strongly believe that you need to get stronger. And I know just the way to do it, endurance running. You got to get some mileage. If you are able to get a 15-25 minute run/jog it will help you tremendously. But you have to be consistent with it. Do this for about 4-5 times a week so your body stays familiar with this working efficiently. As yo start your run/jog find your pace. It is easy. Start easy and push your self a little. But never too hard. The purpose of this is to be able to get your body to work for a period of time without stopping. This will help you get your endurance. Once you get endurance you should see a big difference in your 400m runs.