this "under-eating" stuff is total bull and dangerous advice.
natural training: short and brutal* (vs. long and moderate) is the way to go.
2 hours with brutal intensity may be possible once in a while when you're fully rested and your circadian rhythm hits a high. such days feel great, but they are rare. don't forget: you should also be able to recover from such a workout in a reasonable period of time (in my case 7 days each muscle).
your growth hormone levels will probably peak within the first hour of intense training. if you train longer than that, you will have to pace yourself. So train like a 100 meter dash sprinter who does not need to pace himself. short and brutally intense. then leave the gym, be productive and enjoy life.
* if I may add: "brutal" does not mean toss around weights and grunt like a lovesick rhino