Missoula Marathon training

I plan to start training for the Missoula Marathon on March 22nd, 16 weeks out from the marathon. I didn't want to post on this until after my appointment with Dr. Reach (my foot and ankle surgeon) yesterday. I saw Dr. Reach yesterday for the first time since my only post-op appointment following my surgery to remove a titanium implant from my left foot. He was surprised to see me. He said, "you're not here for my foot, are you?" He considers my left foot his foot. I wasn't. I was there for my other foot. I thought I had injured it in the same way I had injured my left foot, and I wanted to confirm with him that I hadn't injured my left foot, so that I could start training for this marathon. He did an ultrasound on my foot and was able to say with 95% certainty that I do not have a similiar ligament tear in my left foot, just some attenuation of the ligament. Something has been off with this foot for several months now. I didn't know this before yesterday, but the head of the talus (the top part of the foot) slipped out due to the weakening of the ligament and posterior tib tendon. It causes excessive pronation and a feeling as if I'm not "on top of my foot," as I like to put it. The remedy, he thinks, is to do posterior tib tendon strengthening exercises, so that's what I'm going to do.