20 days seems like a long time. It's variable by a lot of factors, but I though 2 horses pulling a very heavy wagon can go maybe 20 miles a day at 3-4 mph on a route, with a need to rest. Pioneer wagons in the US could also go 20 miles a day on the prairie, at a slower speed but for more hours a day. 400 miles seems far for a tiny village to sell their harvest at a commercial town. Oh well, just a story.