I've never run a lot of it - but just out of Yount is the Silverado Trail. Traffic is not that bad, road is policed fairly well in my experience, the shoulder is fairly wide and clean, not a lot of elevation change, hard to get lost, if you are extra unadventourous, do a few out and backs on it and 20 miles will happen in no time. Hope this helps.

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http://www.fitspeek.com the Fraser Valley's fitness, wellness, and endurance sports podcast

I'd avoid Silverado Trail and 29 as much as possible. Go north on Washington and pop out on 29 for about 1/2 mile. Right on Yount Mill. Take that to Yountville Cross, turn left, then right on Finnell. That will loop you back to Yount Str. Turn right then left on Maddison where you can pick up bike path for a mile. When it deadends turn right, go under overpass and left on Solana Ave. that parallels 29 all the way to Napa, and you can do out-and-back on it.

Otherwise you can take the cross streets and do a square run, either north or south. Yountville Cross will connect to Silverado Trail, which you can go south on to Oakville Cross, which will cut all the way back to 29. You can cross it and run back up Solana.