As a member of the general public, can you walk on the running track for exercise? Does it cost anything to use the track?

Yes, the running track is open for both walking and running at a cost. It costs about $4.

There were people both running and walking on the track on the outer edge of the Oval while I was there.

It's a speed skating center so it's not really set up for walking. I suppose if you have a kid training there, you might be able to walk the perimeter for exercise if that's what you're asking.