1) Be skeptical of everything you read, especially in internet forums

It's not that hard to run marathon distance in a day. You can break it up into several smaller runs, with plenty recovery time between them.
For example: 1 mile warm up jog, 3 mile run, 1 mile cool down jog = 5 miles. Run 5 of those in a day, and add on an extra 1 mile 265 yards or whatever, and you're there.
Since you said you were recovering from stress fractures, this is probably a bad goal for you.
2) Marathons are hell on the body. I haven't done an ultra. Even marathon training took too much time, and I decided it just wasn't worth it.
3) Distance running at a slow pace trains you to run distance at a slow pace. Running short fast intervals trains you to run short fast intervals. Give me a guy who can run 20 miles, and a guy who runs intervals, and tell them both to run 100 miles - they're both going to be hurting.
4) Are athletes today more physically capable, or just more specialized? We do have more knowledge, better medical care, maybe better nutrition... but Carl Lewis doesn't have to go out and chop wood, work out in the fields, carry water from the stream, track an animal through the woods, or any of those things that were fairly common even 100 years ago.
5 + 6)