To get the most out of your training it is best to include a variety of training sessions, each of which train your body in a specific way. This post has a summary of the different sessions and how you can include them effectively in your training. Training plans can be found online or you can make your own up using a mix of the sessions here. One thing to remember if you are devising your own plan is not to have two hard sessions together, always seperate them with a rest day or a day of easy running or cross training. The sessions you include in your plan depend on your fitness, the race you are training for (if any), the amount of time available to train and the length of time you have been running.
Ok size does matter. Portion size that is. Many runners fall into the trap of thinking they are burning more calories than they actually are and therefore increasing their food intake by too much. Some people who run wonder why they are not losing weight or lose weight for a while and then plateau. It can be a Catch 22 situation, you run and then get hungry so you eat more. Many people underestimate the calorific value of meals and snacks so the balance of what we consume and what we burn is out. To lose weight there needs to be a calorie deficit.
When I started running 6 years ago I had never run for more than 20 minutes in one go. I had entered The Race for Life, a 5K run raising money for cancer research and thought I had better get training. My first run out was 10 minutes and I felt like my lungs were about to burst out of my chest. How was I going to manage 5K?