If you your fracture is not healed and you run on it, the fracture could complete which could put you out for significantly longer possibly even requiring surgery if it's bad enough.  Being a quarterback and playing with a stress fracture is COMPLETELY different than trying to run a half marathon or marathon on one (you don't say which you are doing).  An NFL quarterback takes a snap, steps back 3 steps and throws a ball.  He MIGHT have to run 10-15 yards.  MAYBE more.  You are talking about running anywhere from 13.1 - 26.2 miles on a stress fracture which is different and honestly stupid if it's not healed.  You're talking 2000 steps (give or take) per mile where a QB takes maybe 1000 steps in a whole game.  Yeah, totally not the same thing. 

Stress fractures take 4-6 weeks to heal.  Best case scenario.  Calcium and Vitamin D are not going to speed healing but they will help your bones get and remain stronger going forward.  Listen to your doctor.  It WILL be a miracle if you make it to the start line.  Even if you do, sorry, but cycling doesn't mimic running.  If you are heck-bent on trying to keep up your fitness and your doctor OK's it, see if you can do pool running. If you don't have access to a pool with a deep end, see if your doctor can give you a prescription and find a place that does.  Pool running is the closest equivalent to running without the impact and will be what would keep you in the best shape.  But seriously, after my own stress fracture in 2008 took me out of my goal race, I had to keep asking myself "Do you want to run THIS race or do you want to run for the long-term?"  Running on an injury like that can take you out for a long time and when I put it to myself that way, the answer was very easy.  Keep up the cardio and IF AND ONLY IF your doctor OK's you right before the race should you even consider it.

Implied but not asked: how was the diagnosis made?  Stress fractures generally can't be seen on an x-ray until they start to heal a couple weeks later.  Prior to that you need an MRI or bone scan.  So a second opinion may be in order.  Though that still may not get you to the race on time.

Let me emphasize that I don't recommend running on a stress fracture for the reasons stated by an earlier poster.

Len

I'm more worried about the arch collapse you described earlier, than a  tiny fissure in the bone's surface. Pound for pound, healthy bone is  stronger than untempered steel, which is over 3 times stronger but  weighs 4.5 times more. The reason is the layered, tubular structure of  bone, called the Haversian Matrix or System:

As  you can see, even a crack on the surface is likely to have nothing to  do with the integrity of the bone underneath. In the popular culture, we  tend to think of bones more with a crystalline structure like porcelain..  which I find comical, especially among runners, who rarely encounter  the shear forces of other sports like football. Brittle bones or  softened bone are something else entirely, in which case you'd have much more to worry about, so make the most of this checkup.

Meanwhile,  the shape of the arch is maintained, not by metatarsal bones and  ligaments alone, but by the plantar fascia and muscles/tendons acting on  the arch, most notably the Tibialis Posterior, which when released  without contraction allows the arch to fall. Even if the muscle (which  resides behind and between the bones of the lower leg underneath the  Soleus and Gastrocnemius calf muscles) is OK, the tendon wraps  underneath the inner ankle bone (medial malleolus) on its way to the  spot that hurts. If that tendon were pulled or detached there could be a  structural fault or neuromuscular release of the T. Posterior,  resulting in the collapsed arch. This should be taken into account  during the diagnosis. In the case of a suspected stress fracture, even  x-rays show little more than surface calcification, which can be due to  other things, including tendon separation or damage.

I  recommend the following Medscape link for both you and your doc, because  it discusses several separate or concurrent pathologies that are  frequently misdiagnosed...http://emedicine.medscape.com/article/386322-overview

Now that you mention the 3rd metarsal there is also the possibility of a neuroma, which is very common among distance runners.

Check this link to compare symptoms... http://www.podiatrychannel.com/mortonneuroma/index.shtml

I've been dealing with this problem for some time (it ruined a ferw marathons), but have managed to get around it by modifying my shoes and taping my toes. There are other strategies if it gets out of hand, but the link will describe them in greater detail.

BTW, if that's what it is, it feels like someone drove a spike into your foot. My first suspicion was a fracture too, but it came and went over years depending on mileage. It is more likely (from a runners perspective) if your shoes are cramped or you overpronate.