mirror modifier for animations? I'm a bit new so maybe this feature exists but i couldn't find it.

But while i'm trying to animate simple things such as a walk or run cycle i find myself frequently ctrl-c, ctrl(-shift)-v-ing the first frame of the animation towards the end of the animation.

Is it possible, maybe somewhat like the way you use modifiers to mirror meshes to 
 use something like modifiers to mirror/copy your keyframes as well, so you don't constantly hace to copy and paste to update the second half of your animation?
You can use F-Curve modifiers in the graph editor. Specifically what you would want is a cycles modifier with the cycle mode set to repeat mirrored.

Here is the documentation