MySQL for averaging missing dates

The Problem

I was looking to update rows where the date column was NULL by averaging the dates of the nearest id before and nearest id after that row.

In the data below, it would update the row with ID 26 to be 2011-10-19

The data looks something like this:

id     date           title
--------------------------------------------
12     2011-09-01     Example One
23     2011-10-02     Example Two
26     NULL           Example Three
27     2011-11-05     Example Four
29     2012-01-05     Example Five
37     NULL           Example Six
38     2012-02-03     Example Seven
--------------------------------------------

The Solution

UPDATE tableX AS t
  JOIN tableX AS next
    ON next.id =
       ( SELECT MIN(n.id)
         FROM tableX AS n
         WHERE n.id > t.id
           AND n.dateX IS NOT NULL
       )
  JOIN tableX AS prev 
    ON prev.id =
       ( SELECT MAX(p.id)
         FROM tableX AS p
         WHERE p.id < t.id
           AND p.dateX IS NOT NULL
       )
SET t.dateX = prev.dateX + INTERVAL (DATEDIFF(next.dateX, prev.dateX) / 2) DAY
WHERE t.dateX IS NULL

Thanks to ypercube for helping figure this one out!