When I started running someone gave me some wise advice. She said when you start, go for however long, in time not distance, you can go. Take note of that. For that week, run three times that same time. Even if you feel like going longer don't that first week. The following week add 10% of time to your first weeks time. Repeat the process of running that exact time three times that week. The third week, increase by 10% and so on. This was the process I followed until I got to the 4 mile mark. It took me a several months to get to 4 miles. Overall getting to 4 miles was harder than eventually doing a marathon. This approach worked for me. Other than that I don't have any research to back this up. I'm not an expert, I'm a Most Unlikely Athlete.