50 miler in 10 months

So I want to do a 50 miler. The problem is that I’m not used to running.. I ride bikes and lift weights. I would consider my shape well past average. I ran 15K the other day in 1.27. No problem at all. My cardiovascular system has no problem at all doing that distance. I could have ran 3 hours without my cardiovascular system being very much affected. The problem is that my ligaments and muscles are not used to the load, so my knees hurt now. I know that I need to step back and build up. But how do I do this. Where do I start? If I had no issues, I would just run 50k a week. And is it even possible to get ready for a 50 miler in September?