Ton,
Training plans vary by intensity, duration, frequency, and specificity. When you compare the 80/20 50K and Marathon plans, you’ll find the intensity to be the same (80/20!), and the duration almost the same (the 50K plan averages about 30 minutes more per week).
The difference is in the frequency and specificity. We wrote the 50K plan for 50K events, and the Marathon plan for Marathon events.
However, you can absolutely use a 50K plan for a Marathon event. I would not recommend a Marathon plan for a 50K event.
David