The West Side of the Reservoir is approx 1.5 miles from the parking lot out to Oak Grove Church Rd. So roughly 3 miles there and back or you can make a full loop for a little more than 3 miles also by following Oak Grove Church Rd a little ways just past the guard rails to get back on the trail on the east side of the Reservoir and the cross over the spillway back to the parking lot. Various little loops exist where you can add to those totals as well possibly.
I'm showing 3.3 miles total on my Strava tracks. 1.5mi on the west side is very well marked, but still has some unmarked trails to explore. The east side is not very well marked and has extra trails and switchbacks which may make it up to 2miles if you explore all of them. There are also trails to the east and west of the Greenway just south of 98. Strava heat maps can show you what's popular over there.
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