It's my new favorite city park (still a long list to get through). Lush, shady, with a winding path that runs along a creek with mini waterfalls, ponds, and lots of sniffable plantlife. The trails open up into a wider area for picnics and play. Lots of pups there; we even saw a couple of gorgeous black and white Borzois.
I'd give it a 4/10 on a scale of strenuousness for dogs. Chaucer's getting up there (he'll be eight this year), so he was huffing and puffing a bit as we jumped around on the creek beds. But the trails are generally flat with mild inclines, and I don't think any dog would have trouble keeping up.
That last pic is actually from Barnsdall Park, which we checked out as well. I don't particularly recommend it for dogs. Not much space, steep inclines, and mostly buildings. Feels more like a community college than a park.