I don't know if this will help assuage some of the stress and anxiety related to corral placement, but I'll give it a try...
For reference, I am a back-of-the-packer, have been for almost all of my many rD races at WDW. I am not a patient person, I have some social anxiety, and I hate crowds with every fiber of my being. I'm not exceptionally slow, but I'm not what anyone would call "fast": if I put my mind to it, my running pace is around a 10:00/mile pace. I do run-walk intervals because I'm old, a former athlete plagued with chronic injuries, and I stopped being competitive when it stopped feeling fun, so my average run-walk pace for a marathon is about a 13:30-14:30 mile without stops.
I personally find many of the accounts of what it's like to be in the back corrals to be grossly exaggerated. You can see by my signature I've done plenty of races at WDW, and here's my average experience in the WDW half and full, starting from the back of a corral in the back 25% of corrals:
Start to mile 1: congested - tight, but not impossible to move near my pace, usually about :30 slower, which gives me time to warm up
Mile 1 to MK: roadway plenty full, but sufficient room to go my pace with little dodging and weaving - I may need to skip a run or walk interval here and there, but I've never been unable to run
MK: a little congested - I can run, but opt not to through most of it because I want to enjoy the park time
Cone Alley to Bear Island (marathon): TIGHT. No way to go any faster than the prevailing speed of the crowd, so I just relax and roll with it
World Dr. to exit (half): wide open roadway - plenty of room to move
AK: a little tight, but I again opt to slow down and take it all in - I could move faster if I wanted to
AK parking lot to WWOS: open roadway - pretty full, but ample room to move
WWOS: TIGHT. I'm sick to death of people by that point and frustration kicks in - If I can talk myself into a happier state, it can be a fun place to people watch, help someone struggling, contemplate all the things I could be doing other than running...
WWOS exit road to DHS: open - smooth sailing
DHS: pretty open - no massive jams that I recall
DHS exit to EP: in other races, monstrous, but not so bad for the full - I think us slow folks are pretty well spread out by then
EP to finish (marathon): who cares?? you're almost done! Also, beer/treats. But really, not a log jam.
EP entrance to finish (half): who cares?? you're almost done! A little tight, but one could totally sprint to the finish if one really wanted to
I think the key here is adjusting expectations. No, I do not think someone why desperately wants to run 7:00 miles from the back of H is likely to have a good time. But if that same person goes in knowing there will be slow sections, and decides to enjoy the experience, there is absolutely no reason he/she can't have a great race!