But, why the marathon?

I expect they targeted the finish line of the marathon because they knew that it would be televised and that lots of spectators would congregate near that finish line.

Luckily, one of the three bombs that were planted didn't go off and will hopefully provide the evidence needed to track down who did this.

A fire in the John F. Kennedy library in Boston occurred at about the same time, but has since been determined to be completely unrelated. That was just a coincidence.

I kinda doubt this would be an international terrorist attack. Even international terrorists would be asking themselves "But, why the marathon?" since there's absolutely no political, economic or military significance associated with it. It's just an easy target because it stretches along a 26 mile route, and so it's not plausible to guard all 26 miles. Still, the finish line should have been recognized as a potential target because of the television cameras that would be located there.