Mukilteo native here.
Depending on how fit you are, perhaps start with "local" day trips hiking or trail running outside Granite Falls off of Mountain Loop HWY.
Beginner mode: Mt. Pilchuck(45min drive to trailhead) The actual trail is 5mi out and back and only about 2k elevation gain, with a somewhat rewarding view at the summit. Its by far the most crowded trail between Granite Falls and Darrington, so don't get turned off if you run into a bunch of folks, especially this time of year.
There's a damn-near endless amount of places to explore in the NW. I ran/scrambled/climbed/snowshoe in the Cascades alone, multiple times a week, every season, even in the worst conditions for ~12yrs and never got bored.
Off of Mt Loop Hwy you'll be able to train and/or explore often, then spend the weekends or w/e in the N.Cascades, BC, I-90, on the peninsula and so on.
For ideas on where to go...
http://www.wta.org/http://www.summitpost.org/http://cascadeclimbers.com/http://www.nwhikers.net/Drink water
