as a coach I admire athletes with clear goals, and also with the courage and determination to pursue them.
usually, these goals are kept private or shared only with the coach and a few carefully chosen friends and family. I think it both curious and brave of her to have made hers so well known and so clear and definite. it's an odd choice.
to answer the OP's question, my understanding is that to qualify for the OT marathon all she has to do is run sub 2:37. last year, 397 women went sub 2:37 in the marathon, which suggests to me that, considering she is already an Olympic champion, and that she is aiming to be an Olympic champion again, her immediate target of 2:37 is, or at least should be, relatively soft. as per the USATF regulations, she has until 19 Jan 2020 to achieve the standard. common sense would suggest that as long as she has her qualifying time by around April 2019 then she will have plenty of time to come down, and then prep for the trials in Jan - Feb 2020. her goal this weekend is therefore to qualify for the trials. anything more than that is a bonus.
however, she didn't need to go run some big-name fancy marathon with loads of public relations and media visibility, all she needed to do was go to some low-key local marathon with a USATF certified record eligible course and churn out a 2:35. but then low-key and Gwen Jorgensen do not very often appear in the same sentence together so it was pretty much a given that she would debut in the full glare of the media and public spotlight. hence, Chicago. another odd choice.
obviously, to make the team she will have to run considerably faster than 2:37, but how much faster is an important question. six months ago Jordan Hasay was in that conversation and I think it interesting to consider whether she still is, and whether she will be again come next October.