I always find it easier to Substring
twice when trimming from both sides of a desired string
string myString = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. <!--extract-->Sed nec luctus magna, ac tincidunt metus. Maecenas sodales iaculis ipsum sed blandit.<!--/extract--> Quisque tincidunt nulla sapien, non faucibus turpis aliquam at.";
myString = myString.Substring(myString.IndexOf("<!--extract-->") + "<!--extract-->".Length);
myString = myString.Substring(0, myString.IndexOf("<!--/extract-->"));
Or, to do the same in one call (uglier)
string myString = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. <!--extract-->Sed nec luctus magna, ac tincidunt metus. Maecenas sodales iaculis ipsum sed blandit.<!--/extract--> Quisque tincidunt nulla sapien, non faucibus turpis aliquam at.";
myString = myString.Substring(myString.IndexOf("<!--extract-->") + "<!--extract-->".Length, myString.IndexOf("<!--/extract-->") - myString.IndexOf("<!--extract-->") - "<!--extract-->".Length);