I found the perfect tool for this a decade or so ago: workrave. It works on all major platforms. The only thing you must do is to not cheat.
But I'm basically repeating what others said. Sitting behind your keyboard and coding 8+ hours a day is bad for your body, and I haven't even scratched the surface. If I have 3-4 weeks with 30+ hours of coding, I start looking at what I'm doing wrong.
Keep moving. Literally.