My dd10 is in a robotics team. During competition season they work upwards of 20-25 hrs a week. Sometimes 8-10 hours straight. The kids, however, don't see this as work. When it is time to go, they don't want to. I think this is what an extra-curricular activity should be like. It should be hard work, challenging, but also bring them joy and not feel like a punishment. There are so many things that can provide challenge and teach perseverance, why not let them choose how they learn those lessons?