The Long Walk is set in the same universe as Nineteen Eighty-Four.
The Major is Big Brother and being "Squaded" is being taken to Room 101. The dark figure Garraty sees is his father's spirit.
One of the people Garraty talks about during the Walk is his father, who was Squaded for anti-Long Walk talk. When the dark figure appears, it's standing and motioning for Garraty to come closer, but it's crying at the same time. Given how Garraty's father hated the concept of the Long Walk, and knowing that his son's going to join him... The Long Walk is a ceremony reenacting a death march from WWII.
Which was evidently a lot worse—the Germans bombed the US East Coast. (It may even have been a death march the Nazis inflicted on U.S. POWs.) Stebbins was poisoned.
When it looks like Stebbins may actually win this thing, the Major has Stebbins' food belt or water poisoned. He doesn't want it to come out that he has an illegitimate son, and doesn't want to take care of Stebbins.- The Major, however, apparently has dozens of illegitimate children, and Stebbins is aware of his half-siblings. Either the Major is going to have to silence many more people, or it's not as much of a secret as he thinks. As for not wanting to take care of Stebbins, the Major probably knows that even the winners tend not to survive for very long after the end of the Long Walk, so it's unlikely that he would have gone so far as to poison Stebbins.
- Alternate theory. Since Garraty was the favourite to win, the Major poisoned Stebbins to allow the masses their victory to keep them distracted from the oppression.
Garraty died as well.
Upon going insane from exhaustion, Garraty lost his mind and thought he still needed to keep walking, despite the Major trying to stop him. The final line indicates he found the strength to run, despite almost surrendering to Stebbins just before. My theory is, much like buying a ticket or being squaded, the line was a metaphor for his death. He was either killed for insubordination against the Major or he dropped dead from exhaustion. The figure at the end was there to kill Garraty
Earlier in the story, Patrick mentions that there really isn't any prize and that the winner is just taken out behind some building and shot. I think that the dark figure Garraty saw at the end was the person who was waiting to do this very thing and Garraty realized this. Since, by this point, he's half insane, had just witnessed masses of other people killed (including people he grew fond of, over time) and extremely tired and weary of pretty much everything, he actually embraces death, at this point, and runs to the person, knowing what it brings. The Long Walkers are all subtly mind into volunteering by the Government
The kids aren't really volunteers. Yeah, the public believe so, and even the kids themselves believe so, but it is actually some kind of subliminal suggestion planted in their minds.The idea that so many people would volunteer without fail every year for a competition like that doesn't jibe.It's not like they are all starving and need the money to survive or something.Many come from tolerable, if not comfortable, backgrounds.Besides, several of the boys remark that they have no idea why they joined. It's true, they don't.They are being mentally forced to.