We strolled through most of the park except the East Meadow, and we only took the Black Spruce Glen trail to the boardwalk - since the boardwalk was closed for repairs. I'm not sure what the distance was, but the trail sign's estimate of 90 minutes to walk Black Spruce Glen was wildly inflated; we went slowly and took half that time. I would guess it was about 2 miles total, not entirely flat.
Despite it being a rainy autumnal day, the arboretum was quite a pleasant surprise. There were still flowers in bloom, and of course the large number of evergreens are interesting year-round.
Supposedly, the arboretum is good for birding, but ironically the most interesting birding sighting of the weekend occurred near the park, not in it - northern flickers hanging out by the side of the road.