Kudos on quitting smoking and Diet Coke, and on planning on a 5k! Those are all tremendous accomplishments! Dealing with junk food should be a breeze after that
There are a couple of things that you may want to do first:
- Keep an eye on your tracker for a few days, and see if there are any obvious nutrients missing from your usual menu. For example, are you low in fats, protein, or fibre? Each of these are things that will have an impact on whether you feel full and satisfied. If you find that you are low on any of these, start looking at ways to add more in to your planned meals and snacks to see if that eases the junk food jitters.
- Start really thinking about which junk foods you are going for, to see if there is a trend that you can satisfy in a healthier way. For example, if you are always going for something crunchy, then popcorn or fresh veggies might be a good substitute. If you are always going for something salty, then baby dill pickles might be good. If you go for sweet, then fruit with yogurt might work. If it's a hand-mouth thing, then veggies and hummus or yogurt dip might work for you.
You also might find that you feel better if you eat more often during the day. The going for seconds with every meal might be because you've let yourself get too hungry. I work best when I eat 6 or 7 times a day, whereas others do better with just 3 meals. Pre-plan some really delicious snacks - ones that you look forward to - to replace the junk in your desk drawer. It's easier to resist a momentary urge for junk when you know that you've got something really good coming up soon.
Also keep in mind that others in your household may have different calorie requirements than you do, so they might need the second helping of a meal to meet their requirements even though you don't. We didn't figure that out in this house until my partner dropped 10 pounds in a month by unconsciously matching what portions I was eating (and he didn't have it to lose). It may be an equally unconscious thing with you that you feel that you want/need a second helping because everyone else is having one.
For myself, I have really dedicated myself to making fresh, whole foods and experimented with lots of delicious sauces and variations. After only a few months, I have to say that the few junk foods that I've tried recently (as a "treat" - hah!) taste, at best, disappointing, and usually just awful. Really, there's nothing that I can't make a healthier and far better tasting version of.
Good luck figuring out what works for you!