Are there flexible pressure sensors for Arduino that can measure forces of at least 600 lbs?
Sensors such as you describe are usually called "Load Cell".  

At the 600 pound weight, I haven't found any "flexible" load cells. A sensor would have to be quite robust.  

Flexible might not be the best answer.

This load cell page has quite a range of load cells, several thousand pounds, also not very inexpensive. 

This might be difficult to install in running shoes :