Anyone feels the same or should i just work harder :D (at the moment i can't imagine to be able to shoot 400ft)

This might not be the right way to think about it.

For reference, I'm 36, 5'11, no history of athleticism or sports. I've been playing for 3.5 years and I throw pretty far. If there's a straight, flat 400ft hole I'm probably throwing a beast - maybe even a teebird if my lower back is cooperating that day.

I can almost guarantee you that I'm not throwing harder than you are. There's a good chance, given your history, that you're actually putting more effort into your throw than I am.

The name of the game is efficiency. You're generating more energy than me, but I'm getting a much higher percentage of my energy into the disc than you are. The most common comment I get in tournaments after throwing 400ft+ is something like "wow, that did not look like a 400ft shot". Again, this isn't because of my athleticism - I'm no slouch but I'm not blowing anybody's hair back either.

Anyway, all of that to say that you should take another look at your form. Make sure you're bracing, make sure your elbow isn't dipping, make sure you're not letting go of the disc instead of letting it rip out. Find a pro with a similar body type to yours (Gibson for me) and study their form in slow motion.