can you do mythic campaign if you have only done the story on normal?

[turbid vault]

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[frigid idol]

Once you're finished the campaign you can replay any mission on any difficulty. Be aware though mythic missions except the 2 featured don't give any gear so you may want to do those over the next couple of weeks

Featured mythic missions give guaranteed T4 gear (T5 if you're350+). Don't forget to attune at altar. Carry outbreak and you'll be fine, bosses are bullet sponges