I am a new runner (more like slow jogger) and completed my first 5k three days ago (woo hoo!), after working up to it with my own modifications to the Couch to 5k program. My feet and legs felt fine during the run, but in the days after I have had a dull pain in the outer part of my left foot as I walk. It is along the outside bottom of the foot, basically the area opposite the arch. The dull pain is not there if I am standing and just putting weight on it - it only happens about mid way through my walking stride when I am "pushing off" of the foot. It's not excrutiating, but enough to be bothersome. Has anyone experienced this, or have ideas? I'm hoping it's not some sort of stress fracture.
Congratulations on your first 5k, and I'm sorry to hear of your predicament. It's not unusual to successfully complete a training program and get caught up in the excitement of race day, only to find you may have overdone it a bit. Now that a few days have passed, it's time to check in on that foot.
As you can see from the above diagram, the peroneal (fibularis) tendon inserts under the bottom of your foot near where your pain is felt. This muscle helps to pronate your foot while walking and running, and performs this action while you are pushing off.
Here is an interactive visual aid that allows you to see these actions separately by clicking "plantar flexes" and "everts" on the diagram...
It is important to note that this muscle group prevents the ankle from being turned under, and is strained and can be torn when the ankle is forced under. While people rightly fear damage to the ankle and foot bones, it is often the tendons and muscles that sustain painful trauma before there is any danger to the bones, which are much harder to break. Over-pronation, as a repetitive motion after weeks or months of training, can also weaken this structure and cause chronic pain there.
Two major causes of over-pronation would be low arches and short 1st metatarsals, measured behind the big toe versus the 2nd metatarsal...
In the diagram above (from the Triggerpoint Workbook), the B arrow points to the 1st metatarsal, with an "X" on the head of this bone. Compare lengths to the 2nd metatarsal head (A) next door, to get an idea of whether or not you might be over-pronating. "C" is an associated callus pattern, which can be another clue. Please note that toes can be of normal total length despite this unstable metatarsal structure.
Low arches are also a sign, because like the short 1st metatarsal, they contribute to the instability of the anke that can induce over-pronation. A common "cure" for both of these conditions is shown below (also from the Triggerpoint Workbook)...
The circles on the insoles represent tough pads of durable material like a thin gel pad available in many shoe departments, or Scholl's Molefoam, cut to the size of roughly a quarter dollar coin. Professional orthotic versions are available from some podiatrists and online. For the brave, these pads can be extended under the entire big toe. Expect some adjustment time, but the results should be a gradual lifting of the arch, straightening of the foot, less pronation, greater ankle stability, and fewer problems in knees and hips. Large claims, but for those with a short 1st metatarsal these things and more are probable benefits.
My apologies and thanks for your time if your foot is not structured this way, but your problem fits a common pattern of an overuse injury due to this common foot structure. In the general population, about one in 200 people can expect to have a problem involving the peroneal tendon, often at the cuboid bone on the outside bottom of the foot. In the case of athletes and especially newbie runners, these problems increase dramatically. Examine your feet today for similarities to the above material, and please feel free to post any further information or questions. I'll be happy to help with several suggestions on how to proceed.