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идти, ехать - imperfective progressive
to be in the process of one-directional motion

ходить, ездить - imperfective iterative
to perform repetitive or multidirectional motion

progressive:
Я...

«идти»/«пойти» are imperfective/perfective couple of verbs. There is a huge amount of information about verb aspect in Russian. It does not specifically relate to the verbs of motion. Very roughly...