GetSerializedSession

We are running the following code in our .NET application to get a serialized session:

            SessionMgr session_mgr = new SessionMgr();

            EnterpriseSession session = session_mgr.Logon(userName, password, server, auth);

            string session_str = session.SerializedSession;

            string SessionURL = HttpUtility.UrlEncode(session_str);

The serialized session that comes back is 4134 characters long before URL encoding, and over 5000 characters long after URL encoding.  The plan was to use the serialized session in an OpenDoc URL like so:

OpenDocURL += "?serSes=" + SessionURL ;

Obviously, a 5000+ char encoded session is not going to work in a URL.  Suggestions anyone?