I downloaded the results for the elite and top 2000 women (last one finished in 3:39:47), calculated their second half time and sorted the results by the difference between the second and first half (faster 2nd half at the top).
I've included the top 30 results at the bottom.
the top 2 women in this list look like they cut the course somewhere (56mins for 2nd half?? and almost 1hr -ve split)
only 4 women had a greater difference between the 2nd half and 1st half
there are only four women finishing under 3hrs in this top 30 list, Vivian Cheruiyot is only other elite woman.
251 ran faster in the 2nd half than the 1st half.
141 ran more than a minute faster in the 2nd half.
272 ran their 2nd half within one minute of their 1st half.
here are the top 30 results:
name, 1st half, finish time, 2nd half, diff between 2nd & 1st half (secs)
Grigoriu, Raluca (ROU) 2:18:56 3:14:57 0:56:01 -4975
De Bonfils, Delphine (FRA) 2:17:45 3:35:40 1:17:55 -3590
Matthews, Charlotte (GBR) 1:43:40 3:19:32 1:35:52 -468
Gray, Jennefer (GBR) 1:48:41 3:29:54 1:41:13 -448
Ternan, Edwina (GBR) 1:47:36 3:29:26 1:41:50 -346
Coyle, Ciara (IRL) 1:51:45 3:38:11 1:46:26 -319
KOSGEI, Brigid (KEN) 1:11:38 2:18:20 1:06:42 -296
Cowley, Laura (GBR) 1:42:14 3:19:36 1:37:22 -292
Rossi, Sarah (GBR) 1:50:34 3:36:17 1:45:43 -291
Swindell, Jennifer (GBR) 1:51:54 3:39:18 1:47:24 -270
Spencer, Ruby (GBR) 1:37:53 3:11:31 1:33:38 -255
Marin, Laura (CRC) 1:43:37 3:23:01 1:39:24 -253
Murphy, Joanna (GBR) 1:36:34 3:08:56 1:32:22 -252
Walker, Patricia (GBR) 1:42:48 3:21:27 1:38:39 -249
Hutchinson, Catherine (GBR) 1:48:28 3:32:47 1:44:19 -249
Fielding, Kathryn (GBR) 1:48:55 3:33:41 1:44:46 -249
Elston, Vicky (GBR) 1:42:43 3:21:27 1:38:44 -239
Marsella-Watson, Ruth (GBR) 1:51:40 3:39:26 1:47:46 -234
Davies, Bronwen (WLS) 1:47:42 3:31:34 1:43:52 -230
Gutsell, Amy (GBR) 1:48:55 3:34:00 1:45:05 -230
Murray, Lauren (GBR) 1:47:02 3:30:16 1:43:14 -228
Oldroyd, Hannah (GBR) 1:24:31 2:45:39 1:21:08 -203
Biddle, Laura (GBR) 1:43:01 3:22:42 1:39:41 -200
Moroney, Janice (GBR) 1:49:47 3:36:14 1:46:27 -200
Stygall (Nee Bissell), Dawn (GBR) 1:46:14 3:29:11 1:42:57 -197
Vebster, Sanja (SLO) 1:50:57 3:38:44 1:47:47 -190
Mora, Noelia (ESP) 1:41:24 3:19:40 1:38:16 -188
CHERUIYOT, Vivian (KEN) 1:11:38 2:20:14 1:08:36 -182
Hunt, Laura (GBR) 1:46:29 3:29:57 1:43:28 -181
Bogantes, Diana (CRC) 1:25:04 2:47:08 1:22:04 -180