Most marathons will allow you to switch your registration to the half-marathon if it turns out you can't do the full distance.

+1 on seeing a PT. Your chiro should be able to refer you. Ultrasound is great for getting swelling and inflammation out of muscles, but I'm not sure what good it does directly on joints. Make sure you're doing your myofascial release and hip strengthening work - a lot of chronic knee problems can be misalignment caused by trigger points mostly in the lateral muscles, and weakness in the hips.

IANAD, etc., etc... Good luck.