I've been playing some days with Poser: I found extremely hard, and unsuccessfull, to create an animation of a character running around, changing path etc.

I'm not using the walk designer, and I created my own running loop, which works fine. Problems arise when I try, for example, to make the character turn 45° and run in the new direction: the animation looks mechanic, I have to rearrange dozen keyframes trying to tune IK's to get a smooth movement around the turn corner. No luck. I have the feeling that while Poser its great for creating simple animations, it lacks features that would allow to easily and quickly repose the character without fiddling on dozen keyframes.

You Poser gurus out there prove me wrong, please: in that case, there are any tutorials that show how to smoothly animate a figure running or walking a complex path with turns, change of directions etc. ?

You can try with IK off .    Fiddling maybe 6 keyframes sounds about right for a fast change of direction while running , you have one foot planted as the pivot point and it works from there.

You might find some animated poses or bvh files of thos actions which speeds the process. Have a look at aniblocks for Daz studio and see if that has what you want.  You should be able to export the scene from Daz .

Anything on keyframe animaation should apply in Poser

post the prelim animation on youtube, that we might see what's specifically wrong with it.  I dunno of any tutorials, other than geep's, wolf's or phil's, but they may not specifically apply to this problem.