From "Neputând merge mai repede, am alergat." I personally get to understand something like: "Not being able to walk faster, I decided to run."

From "Neputând merge mai repede, alergam." I personally get to understand something like: "Not being able to walk faster, I was running."

Native Romanian with not so good English. Sorry for not giving an Yes/No answer; I still hope that the insigt was usefull.

I don't think so, because this action took place at a later time, after a triggering previous activity.

In this context, that would mean "[...] I would run" / "[...] I used to run".

Short answer: yes.