As the king of over packing, I have some light sneakers, cheap sandals, and SPD shoes.
Heavy? Sure
Comfy? Oh yeah.
I can't speak for spd sandals, but I love this combination on a long tour. Sandals for post ride and nasty showers, spds for riding, and sneakers for walking on off-days (hiking or to the bar) and if I'm eating dinner at a resturant.
Have fun with the north cascades, did em about a month ago now. Careful of Washinton and rainy pass, if you leave late, you'll be fighting darkness, lot of riders in pretty good shape have had issues. Beautiful though.