Hi Karl -
I feel your pain - literally! I have an OA knee too that was detected after I slipped, and slammed my knee into a bench, damaging the meniscus. If you have bone-on-bone action going on, I would guess your marathon days might not happen. But, you still may be able to run shorter distances, listening to your knee and practicing very good form. Have you looked into getting an injection like Synvisc (for example)? That would provide some cushioning and allow you to continue to be active. Some insurance companies cover the cost more than others (I was lucky as mine covered it completely).
In any case, you need to listen to your knee, keep up (or start) a good core strengthening regimen, and let your knee tell you how far you can go, if at all. You also might consider running on trails, if they are available to you. The surface is not only softer, but also varied enough that different muscles get used compared to running on cement. That will help keep the damage and pain at bay. And remember not to step past your knee, and land behind your hips!
Keep us posted on how you are doing!
Certified Chi Running and Chi Walking Instructor