Use the lag() function, over the whole result set:
step | end_at | start_at | duration
------------------------------+---------------------+---------------------+----------
Startup | 2011-10-01 02:00:00 | |
Find Unique Personas | 2011-10-01 02:01:13 | 2011-10-01 02:00:00 | 00:01:13
Calculate Loyalty | 2011-10-01 02:01:41 | 2011-10-01 02:01:13 | 00:00:28
Calculate Social Impressions | 2011-10-01 02:02:38 | 2011-10-01 02:01:41 | 00:00:57
Calculate Feedback | 2011-10-01 02:03:09 | 2011-10-01 02:02:38 | 00:00:31
ANALYZE | 2011-10-01 02:09:27 | 2011-10-01 02:03:09 | 00:06:18