Inline editing TextBlock in a ListBox with Data Template (WPF)

Using WPF, I have a ListBox control with a DataTemplate inside it. The relevant XAML code is shown below:

<ListBox Name="_todoList" Grid.Row="1" BorderThickness="2"
     Drop="todoList_Drop" AllowDrop="True"
     HorizontalContentAlignment="Stretch"
     ScrollViewer.HorizontalScrollBarVisibility="Disabled"                 
     AlternationCount="2">
     <ListBox.ItemTemplate>
         <DataTemplate>
             <Grid Margin="4">
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition Width="Auto" />
                     <ColumnDefinition Width="*" />
                 </Grid.ColumnDefinitions>
                 <CheckBox Grid.Column="0" Checked="CheckBox_Check" />
                 <TextBlock Name="descriptionBlock"
                            Grid.Column="1"
                            Text="{Binding Description}"
                            Cursor="Hand" FontSize="14"
                            ToolTip="{Binding Description}"
                            MouseDown="TextBlock_MouseDown" />                      
             </Grid>
         </DataTemplate>
     </ListBox.ItemTemplate>
</ListBox>

What I am trying to do is make the TextBlock respond to a (double)click which turns it into a TextBox. The user can then edit the description, and press return or change focus to make the change.

I have tried adding a TextBox element in the same position as the TextBlock and making its visiblity Collapsed, but I don't know how to navigate to the right TextBox when the user has clicked on a TextBlock. That is, I know the user has clicked on a certain TextBlock, now which TextBox do I show?

Any help would be appreciated greatly,

-Ko9