Which is this XBO we are talking about?
Is LastUpdated an extended field at root level? If not, do you have any standard field with same name at Root level?
If you want to store this duration, then you need an additional extended field defined in your xbo and then refer it in your code in place of this.LastUpdated but if you don't want to store and just use for processing, you can use implicit declarations like,
var LastUpdate = this.LastActivityDate.Delta(Context.GetCurrentUserDate()).ConvertToDays();
In this case, datatype of LastUpdate variable is automatically set to Duration!