How do you say

He plans to run a marathon in three months.

..in Chinese?

他打算三个月后参加马拉松比赛。

tā dǎ suàn sān gè yuè hòu cān jiā mǎ lā sōng bǐ sài 。

How do you say "I bought a new dress and I’m going to wear it for a month." in Chinese?