Before we get to run length, there's one important thing to remember about weight loss: you can't leave your diet out of the equation. We can estimate that running burns about 100 calories per mile, according to Elizabeth Lowden, MD, medical director of medical weight management at the Metabolic Health and Surgical Weight Loss Center at Delnor Hospital. That's a huge generalization, she noted, but you can use it to see that you'll have to run a pretty long distance to really burn calories. For that reason, Dr. Lowden said, "Running as the only change in your lifestyle is less likely to make as significant an impact as running and working on dietary habits." Working on both of them, in tandem, will be much more effective.
How do you do that? Weight loss isn't all about calories in and calories out, but you should still try to eat at a slight calorie deficit, consuming fewer calories than you're burning. (It's best to visit a registered dietitian to see exactly how to do this, but you can also use this equation to get a good estimate.)
You should also look at what you're eating, specifically aiming to eat more whole foods and vegetables and fewer sugary products and processed, packaged foods. Drink plenty of water (especially if you're running), and up your intake of protein and fiber as well. Here's more on losing weight and fat through diet.
So, keeping in mind that diet plays a major role in shedding pounds, let's talk about how you can optimize your runs to lose weight.
Related: If You're Running to Lose Weight, These Are the 4 Post-Run Snacks a Dietitian Says to Eat