Agree that Llanberis route would have the best surface for running - it is quite a straightforward up and down. Given your recent chalenge you should have no problems at all. Having said that, I managed to trip over when jogging down from the top in boots/ backpack, and managed to cut my head.
Some of the other routes can be a lot more challenging and some involve scrambling. If you don't mind heights then the Snowdon Horseshoe is a great route but will have limited running potential.
If you do run the Llanberis route its about 10 miles there and back, so not a particularly long run or walk/run. Its not that interesting as well, unless you enjoy watching the couch potatoes go up in the train. But for a pure run up and down its the best route.
First time I walked it was age 8. I was so proud until the day after when it was reported that someone had gone up and down on a pogo stick.
Edited: 29/06/2012 at 19:31