It is safe to say that DATHAN RITZENHEIN has been tragically underdeveloped in the marathon under Alberto Salazar.
This is guy with a 12:56 and a 60-flat half marathon in 2009 off of Brad Hudson's training (went to Salazar in summer 2009), followed by years of stagnation on the track. His 27:22 10,000 best came in a championship race that same year; he is (or was) certainly capable of sub-27.
Ritz had a 2:10:00 marathon before he went to Salazar, and following three years of no PRs at any distance after that great 2009 season, Salazar finally got him to 2:07:47 in Chicago. This is still a MASSIVE UNDERPERFORMANCE for his ability, and he somehow went backward in 2013 with his only marathon being a full two minutes slower in good conditions on the same course (2:09:45).
What would Ritz be doing for the marathon with YOU as coach?
The McMillan calculator projects a 59:53 half and a 2:06:01 marathon from a 12:56 5,000 so I think that under 2:06 is very reasonable.
I'd have him around 2:05:20.