Something went wrong in your experience. Boars breed extremely fast. You don't even actually need to be there much. Stick around for 2 or 3 minutes, and they'll get knocked up, Go off farming whatever, and when you come back to the pens and the area loads, they'll start popping babies.

So, figure out what you're doing wrong. One possibility is that you have a torch nearby. They're not good at breeding if they're running around scared of the fire. Another is that the area is too big, and they're always hanging out on opposite sides, though I imagine you'd notice that. The last option I can think of is that you just got a bugged boar.

Once you figure out what went wrong, you'll find breeding is pretty easy. However, I do recommend taking the time to find a 2-star boar to breed, as it really makes it much more worthwhile. I have 8 little pens with 2-star boars and I'm swimming in meat any time I slaughter the extras reducing each pen back down to a pair.

(edit)
Also, yeah, don't feed them too much at once. They'll only mate and give birth while you're in the area, but they'll eat constantly. Once they've mated, the timer on the pregnancy will continue without you being present, which is why there are often mass births when I return to the pens. Just show up, give them a lit bit of food, stick around long enough for them to get busy, and you're set. Later on give them a bit more food, and watch them pop babies and get busy again.