Runners need more fuel
I'm gonna deviate from my normal weight loss tricks and how to keep drinking beer and stay fit for a minute and talk about something that will likely be encountered by someone if they move on from initially joining the fitness world, to attempting to increase their performance at certain "sports." I put "sports" in quotations because I don't really consider running or riding a bike to be a sport if you are not competing against anyone. It's more like exercise but you opponent is probably the biggest thing standing in the way of anyone's performance: You.
So when you do finally get past the weight drop and start to focus on things like being able to run a 5k 10k, or even more, you are gonna need more fuel that you normally would. If you have just dropped weight this might seem like a silly thing, to eat a specific kind of food in order to prepare for an activity, but it is going to you and edge if you do.
For runners to perform at their best, the caloric mix should be around 65% carbohydrates but this doesn't mean you should load up on cakes, pies, mocha frappachino, and potato chips - you need to be a bit smarter than that.
You need to be looking at whole grains such as bread or pasta and since these are both carbohydrate loaded, it shouldn't be too difficult to achieve, it is a better idea if you aim for the wheat versions of these things. You can supplement your carb needs by eating fruits such as oranges or bananas and also add in some nuts like almonds.
You also want to add in some protein for long -burning fuel and the right kind of protein would be eggs are lean meats such as salmon. If salmon is too expensive or you don't like fish, then really any meat would be fine as long as you try to get a less fatty version of it. Chicken is always a pretty safe bet as long as it isn't deep fried.
Now timing is key here and everyone is going to be different as far as this is concerned. If you eat too long before your run, you run the risk of having an empty stomach and running out of fuel, if you eat too close to when you are running you run the risk of getting cramps and also those annoying "side stitches" that seem to plague all runners at one point or another.
Since I don't really have a system for how to guess the sweet spot for this, I will normally eat 2 hours before I run, and then will have a small snack just to top up right before I go. This will be a small yogurt cup with some nuts or something like that.
You can always get one of those energy bars as well but you need to be careful since many of these will have processed sugar in them and this is never a good idea - even for someone super fit.
I know that the above advice goes against almost everything that I have said in the past about losing weight, but that is a totally different situation. If you were successful in your losing weight strategy of staying away from basically all carbs, you have probably already dropped most of the problematic weight and are ready to move on to the next phase which is performance on an athletic level. Performance is not just driven by muscle but also by the fuel you put in the tank. A supercar isn't going to perform without the correct fuel and the same is true for your body.
To determine my carb / protein whatever nutritional mix of what I am eating I use My Fitness Pal which is a wonderful program that has virtually all foods and their nutritional information already programmed in by users. Strava is an app that will track your performance over time and is a fantastic tool to see how you have progressed with your running over long periods of time - plus you can show off to your friends. Both of these apps are free.