Never like this year London Marathon can be considered the real World Championships.
In my long experience, I never saw a field like this, both in men and women.
Never in the pasti t was possible to put together , in the Men race, the World Record Holder (Patrick Makau), the  Olympic Champion (Stephen Kiprotich), the multiple World Champion and Olympic silver medal (Abel Kirui), the last winner and bronze Olympic medal (Wilson Kipsang) , the winners of the last two jackpots (Emmanul and Geoffrey Mutai), and other 3 athletes under 2:05, among them one of the most consistant runner of the last years (Tsegaye Kebede) and a former multiple winner of the same marathon (Martin Lel).
To do a preview is really impossibile.  With such a field, of course the organisers look for a possible WR, if the conditions will be ok.  I think the possible split at HM for the best group ever of top runners can be about  61:45, looking at the WR with a negative split coming from a fast part of the race between 25 km and 35 km.
The work of pacers can be fundamental for a new WR.  We need atlete able themselves of times about 59:00 in HM and 2:06:00 or less in full marathon, for being sure somebody can arrive till 32 km.  After that distance, I think the atlete themselves can try some solution, pushing the pace under 2:55 per km.
In this group there are atletes shpwing several times, in the past, their attitude of front runners.  We think of Geoffrey Mutai, never fearing to push at his limit (remember Boston and NY in 2011, when Geoffrey was in his best shape ever), of Wilson Kipsang (what he did last year in London was wonderful, what he did in OG was crazy…), of Patrick Mutai (a terrific 3 km between 26 km and 29 km when bettered the WR in Berlin), and in Kebede (remember his duel with the great Sammy Wanjiru, and more recently his final km. In Chicago last year.
In this group, Abel Kirui and Stephen Kiprotich can have good ciance, also if their times are not at the same level of the other runners, but their ability in big Championships is very clear.
Also about the Women race, this can be considered the best field ever. There are the Olympic Champion (Tiki Gelana), the World Champion (Edna Kiplagat), the silver Olympic medal (Prisca Jeptoo), two atlete under 2:20 like Lucy Kabuu and Florence Kiplagat, one of the fastest debutants all time who is still second in the listo of 10000m (Meselech Melkamu), and especially…. Tirunesh Dibaba.
I think this is the more interesting reason in the ladies race.  Tirunesh has the talent for a great debut, but not always the talent showed in 10000m can be quickly tranferred in a full marathon.  Tirunesh doesn't have really esperience in long distances (she ran one HM only, remaining with the leaders till the last 500m, without doing anything, looking for inning only), and, also if I wait from her a conservative tactic (following the pace of the more expert runners, waiting for the final), nobody can know how much fuel she can have in her tank after 35 km.  Athletes with a long career in short distances, at the top level, have an engine used to go with glycogen (a super petrol), and normally have to spend long time for becoming able to use the most qualified fatty acids as fuel, in order to consum less glycogen at marathon pace.
Also the results of Dubai, with Wude Ayelew (30'11") and Oljira (in the Ethiopian Olympic team for 10000m, 30'26") not in top 10 at the pace of 2:30:00, clearly show that Marathon is a World apart.
Anyway, probably the most interesting and amazing Marathon ever run, and we have to work very hard with the atlete for a matter of respect with the efforts of organisers able to put together the 90% of the top atlete in the World.