Date Duration Numbers

I have a problem, I need to find out how many days there are between 2
date fields and then add all the dates up.

Example...

 name | start_date | end_date
------+------------+------------
 Joe1 | 2002-01-01 | 2002-01-02
 Joe2 | 2002-01-01 | 2002-01-03
 Joe3 | 2002-01-01 | 2002-01-04
 Joe4 | 2002-01-01 | 2002-01-05
 Joe5 | 2002-01-01 | 2002-01-06

As you can see, the duration is:

Joe1 - 1 day
Joe2 - 2 days
Joe3 - 3 days
Joe4 - 4 days
Joe5 - 5 days

Now that I have the duration between the times, I need to add up all those
numbers:

   1 day
   2 days
   3 days
   4 days
+  5 days
----------
  15 days

I can use age() to get my interval (select age(date '2002-01-02', date
'2002-01-01')), but I can't seem to use the field names with that, I have
to manually enter in all the dates.  I have about 900 entries in this
table.  Is there a way to do this?  Thanks...

John