Some random thoughts on the topic:
Hunger is real. It's one of the strongest cravings we have and not surprisingly - if you don't eat, you die. And we've been genetically hardwired for thousands of years to eat when we have, so that we starve slower when we don't have.
Willpower can only go so far. It's a limited and unstable resource. Habit is much stronger, for many of us. You need willpower to start a good habit, but habit creates its own willpower.
Large parts of todays society is geared towards expending as little effort as possible. Yesterday I got off the metro (abandoning a comfortable seat and a good book) to run the last 6,5 km to work. Before I got outside to where I could run I had gone through 2 sliding doors, 1 revolving door and taken 3 escalators, ensuring that my precious resting heart rate was completely undisturbed. After running and showering I sat at my desk for six hours.
Rational eating is just a tiny part of why we eat. I eat what I eat because I like the taste, because it's what's available at my local store every day and every week, because I have cultural preferences telling me that this food is wholesome or traditional, because I want to treat my family to something special and this is the way my mother always fed us when we celebrated something, because I always comfort myself with this treat when I'm feeling down, because this is acceptable food to serve guests etc. etc. And because I'm hungry, or maybe even sometimes - because I feel my body needs this particular food.
Knowledge of healthy food and exercise is important, but I think it's really hard to kick entrenched habits unless you have some insight into why you eat and exercise (or don't) the way you do. For some people those connections are very complicated, for some they're simple and easy to break.
Sorry for rambling on again, I don't mean to sound preachy
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett