What makes Xeno's paradox paradoxical?
The reasoning you gave illustrates why Zeno's example has a chance of counting as a paradox at all. As you show, of course Achilles will overtake the tortoise. But Zeno claimed to have equally good reasoning showing that Achilles never overtakes the tortoise. That's the paradox: apparently good reasoning in favor of each of two incompatible claims.
For Zeno's reasoning and a critique thereof, see sections 3.1 and 3.2 of this SEP entry.