ASP.NET：バックグラウンドで処理をさせるには？

<asp:TextBox id="TextBox1" runat="server">10000</asp:TextBox>
<asp:Button id="Button1" runat="server" Text="Exec"></asp:Button>
<asp:Button id="Button2" runat="server" Text="Refresh"></asp:Button>
<asp:Label id="Label1" runat="server"></asp:Label>

public class Async : System.Web.UI.Page
{
  protected System.Web.UI.WebControls.TextBox TextBox1;
  protected System.Web.UI.WebControls.Label Label1;
  protected System.Web.UI.WebControls.Button Button1;
  protected System.Web.UI.WebControls.Button Button2;
  
  public delegate string ExecDelegate(int duration);
  
  private void Page_Load(object sender, System.EventArgs e)
  {
    if(!IsPostBack)
    {
      Session["state"] = "未処理";
      Label1.Text = Session["state"].ToString();
    }
  }

  private void Button1_Click(object sender, System.EventArgs e)
  {
    //処理クラス
    ExecClass objExec = new ExecClass();
    //デリゲート
    ExecDelegate dlgt = new ExecDelegate(objExec.Exec);
    //コールバックメソッド
    AsyncCallback cbm = new AsyncCallback(CallbackMethod);
    //非同期処理開始
    IAsyncResult ar = dlgt.BeginInvoke(Int32.Parse(TextBox1.Text), cbm, dlgt);
    
    Session["state"] = "処理中";
    Label1.Text = Session["state"].ToString();
  }
  //コールバックメソッド
  void CallbackMethod(IAsyncResult ar)
  {
    ExecDelegate dlgt = (ExecDelegate)ar.AsyncState;
    string ret = dlgt.EndInvoke(ar);
    
    Session["state"] = ret;
  }
  //状態チェック
  private void Button2_Click(object sender, System.EventArgs e)
  {
    Label1.Text = Session["state"].ToString();
  }
}

//非同期処理対象クラス
public class ExecClass
{
  public string Exec(int duration)
  {
    System.Threading.Thread.Sleep(duration);
    
    return "処理完了";
  }
}