When I was around 15 I used to do 10 hours of x86 assembly programming, and then several hours every day after school for a month or so in a row. Parents would have to force me from the computer.

I attribute it to a younger brain, NO internet and NO fun distractions. At 42 I just don't see how I did it, and I know I could never be that focused. Just sitting still for 4 hours make me feel quasy now, and I need to use my physical body in some way.