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Mt. Si is 4 miles and 2900' of elevation gain. Do it 1 way in under 2 hours and it's a great workout. Put 50# in your pack and it's even better.Mailbox Peak was a beast of a hike. I imagine it still is, but I also think they switchbacked it during trail renovations. Tough to access right now due to the paving of Middle Fork Road. Mason Lake is 2400' of gain in a little over 3 miles one way...also a pretty good grind.All 3 are pretty close to Seattle.
If you ever come up north, Chuckanut Mt. has some trails that go UP. Try the Oyster Dome on a clear day & the view is a good reward.