I have done this same route many times, both on dual-sport motorcycles and on mountain bikes. You will be fine. You will see that there is a fair amount of vehicular traffic on the road, even on the holiday. A nice loop is to ride up from Granite Falls, do the Mountain Loop to Darrington, then connect to the Whitehorse Trail in Darrington, which then connects to the new expansion of the Centennial Trail. You can then ride Centennial all the way back down to Snohomish.

Depending on if the County has gotten out there yet, be advised that the gravel portion of the Loop can have washouts and significant potholing early in the season.