Sprained or Broken Foot??

I need some advice on my most recent injury... It's kind of a long tale--sorry!

Five and half weeks ago, I rolled my foot hard walking down the stairs (with some help from our new puppy).  I have twisted my foot hundreds of times, but this felt much worse than usual, and there was quickly a pronounced bump near the arch of my foot.  I went to the urgent care center and got x-rays.  The x-rays showed what looked to be a very old break in the general area (4th metatarsual), but no new breaks.  I have no idea when I might have previously broken my foot--sprained it yes, broken it??  So the diagnosis is a severe sprain.  Oh, and this was the day after I registered for my first marathon, which is in mid-January.

I got in to see my chiropractor/ART guy early the next week, and have been going to see him for ultrasound, electro-therapy, etc, since, cutting back on the regularity as the foot improved.  For about three weeks, I did all my running in the pool.  About 2 weeks ago, I was given the green light to really run, starting slowly (10 minutes).  I have worked up to 50 minutes, all without any pain.  There is still some soreness in a small area of the foot, and when I step wierd (usually when barefoot), it hurts a bit. 

This morning, I am in seeing the chiro/ART guy, they are doing ultrasound, and for the first time it hurts--like a sharp pain that causes me to pull my foot away.  After checking out the machine, the doc notices that they had it set to 'continuous' instead of 'intermittent.'  On 'intermittent' there is no pain.  He says they occassionally see that type of reaction, but usually only in the case of fractures(!), or occassionally in severe tears to the small fibers that connect to the bone.  Doc tells me not to worry about it, I'm doing fine, keep doing what I'm doing.

Should I just keep going and not worry about it?  Or should I go get another set of xrays and a second opinion?  At this point, wouldn't the bone have more or less "knitted?"  (Note the terminology I picked up from breaking my wrist? )  Any advice would be appreciated!