You've clearly never ran 50 km in one go.

You're talking 5 intense months to safely train minimum, for a person who can run 5 km.

Training for a 50 km isn't "just aerobics", it's also about strengthening your bones and ligaments and developing the mental tools you'll need to finish. It's about learning what your body needs to stay hydrated. Learning how to minimize calorie use. Learn how to run through the walls (yes, plural, there's normally two of them on a 50 km run). How to eat on the run without throwing it all up or squirting it out from your shorts. Aerobics is the least of it, IMHO.

12 weeks is a laughable claim.

And yes, I've run 50km (and far beyond) in one go.