Reply by Woodknack

On a table that size with pocket screwed aprons, I too would have added the corner brackets. Intermediate stretchers (what you called runners) aren’t necessary on a table that size if built correctly. Angled braces are amateur hour, only used to repair improperly built tables. If your table is racking, it means the screws are not holding tight on the aprons but the corner brackets should fix the problem.

-- Rick M, http://thewoodknack.blogspot.com/