Get your root
element add new elements to it then save it:
var rootElement = Interfaces.Root;
foreach (var i in intf)
{
var element = new XElement("Interface",
new XElement("name", i),
new XElement("vlan-tagging", XElement.EmptySequence),
new XElement("unit",
new XElement("vlan-id", "10"),
new XElement("family",
new XElement("inet",
new XElement("address",
new XElement("name", "10.10.1.23/24"))))));
rootElement.Add(element);
}
rootElement.Save("Interfaces.xml");