Thank you guys so much, but I still have a doubt.
Ok, a directional verb with по- implies the beginning of the movement: я побежал = I began to run. A non-directional verb with по- implies short duration: я побегал = I ran a little. But, how will I conjugated these verbs? I mean, the conjugation of Побежать (perfective aspect) is different in future tense than the conjugation of Бежать and Бегать in future tense.