I've only done the long route, which is ~80 miles and 10k climbing. How it works is they give you a little card that is like a fake passport, and they give you a stamp at the ~18 stops along the way. At each stop, they have food and water, plus one extra stop in the middle around lunch, so there is plenty of support. I actually made a good decision the last time I rode and brought less food, lightening the pack which made the climbs a bit easier. The first time I did it I didn't know what to expect and I rode with a Camelback - that was dumb. The only downside is catching the ferry in the morning(you should drive on as both ferry docks are a ways away from the start), but once you're on the island everything goes smoothly. It's a fantastic ride, with some really steep pitches. Not sure if you've been out to Vashon before, but lots of 8-12% climbs. Here's the map:
https://ridewithgps.com/trips/40796551?lang=en