Slower running will often make folks gain weight--which seems contraintuitive---but I know it is true. When I trained for marathons, my running partner and I both actually gained weight from doing long 18-24 mile runs-- when we were out there for HOURS, and I immediately lost it after I stopped running long distance--even with eating exactly the same.
It's a metabolic thing.
Your body gets stressed and learns to get more efficient with the calories you do eat--you also gain muscle over time. If you did sprints and faster running, you might lose weight-- so if your goal is to bulk up and/or lose fat... stay with the slower pace, even walking speed.
Running at any speed is great cross-training--be sure to stay hydrated though, so that is not why your weigt shifts--!
