I don't know if this is at all a viable option for you, but some of my favourite memories from growing up were going for runs with my dad. It's not the kind of exercise that's going to get you marathon trained, and my dad was also doing a heap of exercise on his own without me, but if you just wanted to get enough exercise in to feel like you're being healthy, that could be one way to fit it in—with the added bonus of helping your kid's mental and physical health at the same time.