"For nearly sixty years he's walked the County lines doing his duty. Doing what has to be done. Soon it'll be your turn. And if you won't do it, then who will? Who'll look after the ordinary folk?" (1.77)
Hearing this from the Spook, we equate his duty with the same sense of duty that superheroes feel to protect us common folk. Which raises the question, why can't the ordinary folk look after themselves? What's so special about the Spook—about Tom? Or are these two simply willing to do a job that most people would never even consider?
"You've the gift and the strength to do what has to be done. I know you're going to make me proud of you." (1.79)
Here, Mam speaks to Tom's strong sense of duty. It's a nice little pep talk, but sometimes it's hard to tell the difference between Mam's praising Tom and her giving him a guilt trip.
"Hunger's something you're going to have to get used to," [the Spook] said. "We don't each much when we're working, and if the job's very difficult, we don't eat anything at all until afterward. Fasting's the safest thing." (2.50)
It's not just a Spook's duty to protect people. It's also his duty to ensure that he's in the best physical shape in order to help people. We can totally see The Spook Diet ending up on the next Dr. Oz Show.
"We see things that others can't, which is both a blessing and a curse." (4.8)
He had me digging two practice pits a week, which was hard, sweaty work and took up a lot of my time. (7.19)
The word "duty" isn't a very pretty word. (Hold your giggles.) And sometimes, doing your duty means doing difficult, dirty work that goes above and beyond what you might have expected to do. The Spook might call digging these practice pits a "character-building" exercise.
I always had to do things the hard way, so he started me off by making me draw a map of my own. (7.20)
Sometimes you have to do things the hard way, in order to learn the ins and outs of whatever new skill you're trying to acquire. Spooks don't rely on GPS. They use their own smarts, which are always there, even in unexpected situations.
"You're nearly a man now, and a man has to work." (7.59)
Learning to fulfill one's duty, whatever that may be, is an aspect of any coming-of-age story. While Tom isn't quite there yet, he is slowly learning to take on responsibilities that affect other people. And this one of the first steps one must take toward adulthood.
"You're the last chance. The last hope. Someone has to do it. Someone has to stand against the dark. And you're the only one who can." (7.61)
This is another one of those motivational speeches from Mam that sounds a lot like a guilt trip. We think the subtext here is not just, "If you don't do this, I'll be disappointed in you," but "If you don't do this, the whole world would be disappointed in you. You know, if they aren't all dead already."
I told [the Spook] about Billy. He nodded sadly but said not to worry because he'd do what was necessary. (11.141)
Adults have duties too, and the Spook's duties are difficult in a different way than Tom's are. He has had to bury someone whom he mentored, someone who died on the job. Poor Billy. And poor Spook. Digging a pit suddenly sounds pretty easy to us. But, as any good superhero knows, you don't get to choose your duty... it chooses you.
I'm going to try just as hard as I possibly can because I want [Mam] to be really proud of me. (Epilogue.5)
Looks like those guilt trips/pep talks from Mam are working. At his age, Tom is much more likely to do his duty because of an obligation to his Mam, rather than an obligation to some Greater Goods—some bunch of strangers he doesn't even know.