The Darrington-Arlington road, westbound, is not really that bad as far as traffic goes. Yes, there are motor vehicles, and parts don't have over-wide shoulders, but you will probably survive. Note that there's not much, if anything, between Darrington and Trafton (5 miles or so east of Arlington) in the way of food or liquids for purchase. If you don't want to go on to Arlington and pick up the trail there to Snohomish, just past Trafton you can take a left and head down to Granite Falls, and then on to Lake Stevens, and then take the trail.

As you head up the Mtn. Loop, stop in at the USFS Ranger Station at Verlot (after the store) to find out about the Ice Caves trail. That is a pretty short hike, maybe 1.25 miles each way.