Weight gain and loss are governed by hormones, not just calories. Excessive endurance training without adequate rest and recovery can lead to elevated levels of the stress hormone cortisol. Stress—which can come from training hard while still holding down work and family commitments—is a surefire way to make your body hang on to every bit of energy (stored as fat) that it can.
Solution: Take your easy days easy and recover properly. Quality sleep and rest is as important as any workout. Support your body by filling up on anti-inflammatory fruits and veggies rich in antioxidants, and time your recovery meals and snacks appropriately.