The Hardesty Mountain trail is definitely on a mountain! I highly recommend getting a shuttle up the gravel road to the top of the trail, but even if you do, it'll still be a stiff 1-1.5 mile ride/push to the top of Hardesty Mountain. Shuttle to the top using USFS Rd 5835 and then a right onto Rd 550.
From there, be prepared to descend fast & furious back down to the reservoir below! This trail is a brake-burning descent down narrow pine needle-covered singletrack through beautiful a beautiful PNW forest. The upper section features a few serious switchbacks that might require a dismount, but otherwise it is pretty smooth and flowy--but steep!
The lower section of the trail has seen more wear and is a little rougher and a little wider, but it's still brake-burning steep and relatively flowy.
Once at the bottom, you can pop out onto the road, or this trail can be looped with other trails in the area, including Eula Ridge and the South Willamette Trail.