Cross Training - Isn't this kind of late?

So, my last long run of 15 ended up straining my knee. I think I built up a little too much for my own good. It wasn't the 15 miles alone that did it. It was a 12, at 10 and a 15 all within too short of time, and all including hills. I guess I should have known better. However, I felt strong every time!

Anyway, I am in a bit of trouble. I've done some elliptical work - with no pain, but I don't seem to be able to run worth a bean. I ran a 3 miler on Wednesday - through a some very minor pain. Then I did some more cross training, and I received some prayer for my knee on Saturday that felt great. I did another 3 miler on Sunday afternoon - hardly any pain. I kept it short on Sunday, so I could possibly do a more sizable run (hoping for 9) on Monday. When I went out on Monday, however, I couldn't even make 1 mile without pretty significant hurt - so I walked home.

This week I am only going to do cross training. The elliptical doesn't seem to aggravate the injury at all - I don't understand it, but I am going to go with it. I had hoped to get a long run of at least 18 miles in a few weeks before the marathon. At this point, however, I might be wiser to make sure that I can heal up well. I am not sure. I want to be careful, but I still need to build new levels of endurance and strength. This is causing some anxiety. But hey, this is all part of the marathon challenge.

I believe I can do this thing. I am learning to really enjoy running. Yet, I am anxious as to whether I can both nurse my knee and build strength at the same time.