I think the answer to your question is "people don't concentrate for 2 hours at a stretch". That's why the pomodoro technique is so useful!
I'm very focused in general, and I find 45 minutes to an hour is the longest I can sustain my attention on a task, especially if it's boring. Contrary to the other poster I don't think that I have ADHD (though I don't doubt I'd be able to focus for 3 or 4 hours straight if I took dexies, since my husband *does* have ADHD and the medicine does that for him).
You should look online for resources about combating procrastination, getting motivated, etc, as it's the same sort of problem everyone runs into.
Here's my personal productivity tips: