The missing piece in the "if I don't exercise I'll gain weight" logic is that if you don't exercise you'll be far less hungry. When you exercise regularly and intensely, especially lots of hard cardio, you get used to your appetite being roaring and dangerous. It's understandable that if that's what you're used to, you'd be very worried about the idea of reducing your food intake at all. It seems impossible. The thing is, when you're not engaging in loads of hard training, your hunger goes from roaring and dangerous to something that meows and purrs and waits patiently. You find yourself totally satisfied and very happy with considerably less food. If you're eating to your hunger signals, your body will tell you how much. It knows. It's just our thinky thoughts and control freak override tendencies that can throw it off of its game.
I've had several long layoffs due to illness and injury. Every time it happens I lose weight, sometimes quite a lot, that I then have to work to put back on. So, although I like to exercise, I don't worry if I have to stop or cut back for awhile. Even if I'm healthy and training normally, I'm very aware of a sweet spot where I'm enjoying my training, recovering properly, and my appetite is friendly and easily satisfied. If I overdo it, underfeed it, and don't recover, the roaring, scary, bottomless pit appetite comes back. I don't like that one.