Musisz wkrótce zaczynać.

English Translation

You must start soon.

any particular reason zaczynać is used here? couldn’t we use zacząć instead? Since it’s a one time thing, right? It’s starting once, and it’s soon.

Your intuition is right, “zacząć” would sound much more natural in this context. “Zaczynać” is rather awkward here.