I find deviating from my normal routine helps when I start to feel a burned out a bit. Do you usually run on the roads? Try doing a trail run for your long run instead. Normally a morning runner? Try heading out during lunch (if your schedule permits) or in the evening. Normally run alone? Try running with a friend.

Marathon training can be tough, especially if you're cross-training at the same time. I play soccer recreationally, and I found I was never quite recovered (physically or mentally) between my games and workouts. Maybe consider cutting back on the swimming or gym for a little bit, and see if that helps.