A slight correction unless this year brings a change, you will know your race number not the corral with the issuance of the waiver. The corral is on the bib issued at the expo. It's not hard to get an idea of where you are and in the past WDWM has leaked corral info out. Once the waivers aer out we need to set up a waiver thred along with expected times. From there we should be able to get a feel for corrals. In real general terms the corrals have been about 2000-2500 folks - but that was before the realignment of the start.
In all seriousness, jeff limits first timers to 23 or 26 depending on the program and the date issued. I would seriously re-evaluate running 28. It's been my experience with runners that that is an invitation to injury. For a first time runner I would seriously look at a 20 -23 mile cap. I could be persuaded that a longer run is in order if your running log demonstrated several weeks of 50-70+ mile weeks; but most of us are not in that camp.
I tend to agree with Frank that after a few marathons are under one belt that limiting long runs to 3 hours or so is in order. For me I would not run a training run any longer than 3-3:15 any more; but then I also know what demons I will be fighting on race day and have a full tool set of mental and physical work arounds that I know work for me personally.
Just my opinion and experience on the subject - but if I were your coach I would cut your shoe laces at mile 23

. (OK really not). As with all things in this arena, YMMV and the fact that we are all an experiment of one makes no 'correct' answer.