I went on a 6 mile run a week ago and got pretty sharp pains in my left foot (always the left foot!) I felt like I might be getting bone spurs or something that were pushing from the inside-out. But I completed my run without anything catastrophic happening.
Beginning of this week I went on an easy 3 mile run and my left foot was still hurting pretty badly. Same issue. Hmmmm. Still feels like my bones are poking me from the inside out. In retrospect, I suspect I was not really using very good form, too much putting my feet in front of me instead of beneath me. That makes it so my feet and body weight are not coming down towards the ground at a 90 degree angle, but at a sharper angle, which makes the force of my body weight move up the length of my foot. Hard to describe but easy to notice. It has to do with lateral force versus vertical force. I believe my feet get hurt really quickly when I expose them to much lateral force. Good form means not much lateral force and more vertical force, from which my feet do just fine.
So today I decided to just go very slowly and really follow good form on the same 3 mile course. I eased into it gradually, keeping myself strictly to form. Feet placed directly beneath me. Body poised forward, hovering over my thighs. Shins vertical to the ground. Ankles bent, landing on front of feet. It got easier and easier as I went along and pretty soon I was running along at a nice pace, even passing quite a few other runners. Still focusing on form and easy foot-falls.
I got done with the 3 mile loop and decided I was feeling pretty good, so I ran it again. I stopped at one point to work on ankle flexes, which made finishing the rest of the run easier.
Nice thing... left foot came out just fine! Just a slight achiness on the old problem spots, but a whole lot better than my last two runs were!
Bottom line: Form, form, form! You can't take a vacation from good form.