You are using the correct approach. I have slightly modified your code and the text is inserted at the end of the first paragraph.
Here is the code that I am using to insert a text after the first inline
RadFlowDocumentEditor editor =
// determining the inline we would like to navigate to.
var run = document.EnumerateChildrenOfType<Run>().First();
// navigating to the end of the obtained inline
// creating a Run with text and foreground color.
Run myRun =
// inserting the new Run.
Attached you can see a screenshot from the result.
As a condition of determining the inline
you would like to navigate to, you could use the text of a run or a bookmark name. Of course, this should be done according to your scenario.
I hope this information answers your question.
Telerik by Progress