убегать - to be in a process of running away - imperfective
убежать - to complete running away - perfective
убегаться - perfective - to exhaust oneself with running

In the same way:

уплывать, уплыть, уплаваться
уезжать, уехать, уездиться
увозить, увезти, увозиться
etc...


compare to
бегать - iterative (indefinite) imperfective
бежать - progressive (definite) imperfective

There is complicated and flexible pattern including stress. There IS a system, even if it is difficult to formulate explicitly.

BTW here is nice picture about verb aspect.