its bad practice to animate by doing one thing at a time like this without even bothering with any other parts. Sure it looks good now, but when you try to add the upper torso you may come upon issues you could have rooted out earlier before you'd done all that work on the legs, and end up having to do much more. (ask me how I know

)
You should animate the torso and legs at the same time, even if it's just a rough stick figure of the upper half while you work on the bottom, so you get a picture of how everything will move all together.
That said, good work so far.
On the near leg, it never seems to really fully extend, just kind of keeps this slight curve to it.
The far leg seems to kick really hard when going forward. This, along with the last point, is contributing to why I can't tell if he's walking or running. The title says problems running but this looks more like a walk cycle, or something. Run cycles have a point where both feet are off the ground, and in all-out sprints it seems like the air time is far greater than the time contacting the earth. Walks, though, have at least one foot firmly planted at all times.
So uh yeah. Hope I help some.