From your original template in your first post, the XPath to the Name field is: /my:myFields/my:Name. It looks like you have your date field XPaths correct, but you may want to rename those fields eventually. Field4 isn't very descriptive and may cause you annoyances later on.
So, to get the value of your name field:
name1 = MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:Name", NamespaceManager).Value
For your dates, I think you'll need to declare those as strings, get the values from the fields, then parse those values to a datetime data type, at which point you should be able to get the difference for the duration.