I saw your post elsewhere but can't respond there from my phone. To answer the question: Can I just increase mileage on a marathon training plan for a 50k? Yes. That is what I did for my 1st 50k. I don't know what your marathon plan looks like but I would gradually increase the mileage on the mid week run and your weekend long run. Maybe get your mid week up to 10 or 12 and your weekend up to 20 - 24? Remember a 50k is just a long marathon!