...a fairly sedentary life consisting most on fritos, tab, and mountain dew.
Most people in this thread are likely to focus on the highlighted part. And they are certainly good in doing so. Spending 45-60 minutes, 3 times a week, picking up heavy things off of the ground is one of the greatest things I have ever done for my strength and physique. It has been great for building muscle and cardiovascular health. However, when you say "fit" I assume you meant fat loss, first and foremost. And when it comes to fat loss that is done in the kitchen.
Well, here's my angle. Exercise is for strength, endurance and health. That is, when you lift you should be lifting to increase your strength and what you should be counting is the weight lifted and the reps repped. When you cardio, what you should be counting is miles ran/sprinted/biked. However, what many do is count the calories burned instead. And you do burn calories. You burn calories during the activity itself, you usually get a metabolic "afterburner" effect and you burn calories when your body rebounds (this also has the effect of partitioning a portion your dietary protein and fat towards tissue and hormone construction instead of just flat energy). And that's great. But you just CANNOT out train a shitty diet. Saying to yourself that you can eat/drink X today because you did Y is such a dead end, terrible, philosophy that gets many in trouble. The freedom to eat something because "you earned it" just leads to heartbreak for many. If you're going to exercise, exercise for the sake of your body's strength and health, but don't think that it will suddenly make that sugary coffee and bagel a non-factor in your obesity/diabetes.
That said, I honestly believe that the "fritos, tab, and mountain dew" part is the real core of the issue here. Refined sugars and grains coupled with modern fats (seed oils, trans-fats) are the bane of many peoples' lives. Insulin resistance, leptin resistance, celiac disease, IBS, SIBO, etc.. Simply switching to whole foods can almost entirely bypass this issue. Learn to cook your own meats, find tasty vegetable recipes, use fruits and nuts as calorie/nutrient dense desserts. When you do this the trans-fats disappear, the refined sugars and HFCS disappear, the 600+ grams of carbs a day disappear. You will learn the role that protein, fats and carbohydrates play in your body and how blindly trying to cut one of them to zero is a poor decision (seriously, when did we decide that we DIDN'T need dietary fat for healthy tissue and hormone production?). Your hunger will likely diminish as well as these foods tend to be VERY satiating.
It's funny when people ask how to get in shape that they will jump up and be ready to run in place for hours on end per week, but if you tell them that they will HAVE to cook their own meals, well, suddenly they're deers in headlights. People seem more willing to spend hours on end spinning away in their spin classes than spending a few minutes in the kitchen.
Exercise = Strength, endurance, health
Diet = Fat loss, disease control, health
Do them together, but don't think you're going to get strong just through eating or that you're going to lose fat just through running.