Junk food isn't necessary, just (very roughly) 3500 calories per pound that you don't burn off.
You don't have to force yourself to do much. When you're hungry, regularly eat until you're full and then have a bit more, just not enough to feel sick. Your stomach will gradually stretch and not only increase in capacity but demand more food.
If the amount of sugar in your food has serious consequences, then there's nothing for it but to carefully control the amount of sugar in your food. One simple way to help with this is to make simple recipes with two main ingredients, one sweet and one not (e.g. fruit and nuts, or plain batter and syrup) and adjust the ratio until you get something you can eat in large quantities without crashing.
Even if you were to restrict yourself to a raw food diet, it's surprisingly easy to gain weight. Search YouTube for "got fat on raw food" and you'll find an army of very surprised people whose new diet didn't work out for them.
10 years
