For the Crib Goch route, I think a lot of it depends on your balance. If you're used to walking, it probably won't be an issue, but there's always the danger of twisting your ankle running on such an uneven path. If you was going to do this, I'd recommend doing the whole of the Snowdon horseshoe. I'm not sure about running it, especially with the steepness on the peak after Snowdon, but it's a beautiful route. 7 miles of tough up and down with beautiful views.
The Llanberis path is a lot more touristy, but would require a lot less training, despite the longer distance.