if you write all your Runs in the same line, the empty space will go away. Basically a new line here is one empty space on the UI.
<TextBlock Foreground="DarkGray" VerticalAlignment="Bottom" Margin="0,0,0,8"><Run Text="total length "/><Run Text="{Binding TotalHours}" FontSize="48"/><Run Text="h "/><Run Text=":" FontSize="48"/><Run Text="{Binding TotalMinutes}" FontSize="48"/><Run Text="m "/></TextBlock>
