How far from traders hotel in Beijing to riverside hotel in BJ?

There is around 32km between the two hotels and it takes around 60 minutes by taking a taxi if the road condition is not bad.
But you can try to use the local subway and I think the subway system is much easier. You can walk to the Dawang Road Station to take the subway line 1 to Gongzhufen Station and change to line 10 to Xiju Station. Then change to line 14 to Garden Expo Park Station. After that you can walk to your hotel easily.

thanks! any idea how long these trains should take?

The whole journey takes around two hours.