Well, you can see, from the vantage of I-84, a bike path as you drive between Sunnyside and Prosser. It's there, more or less parallel to the interstate. So, we put the bikes in the van and go see if we can find the Prosser end of the path to get a nice ride on a winter-ish day.
We find it. Park the van at the rest stop in Prosser, get out the bikes and start: head out (right turn) of the rest stop (so you're heading NW). Stop at the intersection, and straight across to Wine Country Road. You can see bike path off to the left of the road, and if you just keep pedaling, the path is available on the right in a few hundred yards. Lots of dogs in the yards near the start, all nicely corralled.
The path is a little rough at first. No glass. No visible tackweed. The occasional horse-apple. It's dripping rain and we wander on. We're biking through an agricultural area, but not the growing season. No one is on the path at all. No water in the irrigation ditch that we bridge over. We quickly get to Grandview; advertised population 8705. The bike-path puts you on what appears to be the main street through town. Nice looking town with plenty to offer a Randonneur. We keep riding, and see the path resuming off to the left. Cross over and head to Sunnyside. On this brief stretch the path is furnished with park benches, water fountains and ornamental trash recepticals. There are signposts that count down the mileage to Sunnyside, and we're there. We follow the path through town, looking for the end. Plenty of candidate food stops. The traffic lights respond quickly to our punch of the button for the pedestrian crossing. And then the path ends in a park and ride in Sunnyside, at the intersection of the Yakima Valley Highway and N 16th Street. About 21km from Prosser to Sunnyside. We turn around (into the wind) and ride back.
The path would work fairly well for part of a Brevet - and amounts to a touch over 21km from end-to-end.