Finding equivalents between English and Japanese is quite easy at the start, but it begins to malfunction as you move attention to the "small things." So, it's not a stumbling block only to you. It is, in fact, a hurdle for everyone to be overcome when learning another language.
In this case, "5 years left" or "only 2 hours to go" can be expressed with the use of "mada" and "ato":
まだ5年ある(あります)。あと5年ある。
まだ2時間ある。あと2時間ある。
The difference between the two lies in the shade of meaning.
"Mada" suggests that the speaker thinks there is still enough time, meaning "I have 5 years and that's good enough for me."
"Ato" is commonly used just as a marker for the remaining period of time but also as suggestion of time running out:
あと5年しかない。(There's only 5 years left to go.)
Therefore, まだ5年しかない is incorrect.
HTH!