I tried to go from the Beimen station the other day. It is very simple. It took less then 3 minutes from the platform (up 2 levels) to the connector entrance of the Airport line.

However, the entrance was still closed. The entrance is still 1 big city block from the west building of the Airport Express. So, that adds to the walking but I think it is pretty safe to assume it is an easy walk (air conditioned and accessible).