Not super-common, but fairly widespread and definitely jokey. I'd let it in.

遅いランナー	756
スローランナー	634
亀ランナー	232

I agree this might not be all that well-established, but it's still sees 1:3 of the usage of more obvious and literal expressions meaning 
the same thing. Compare "turtle runner" and "slow runner" in the English (2019) corpus on Google Books ngrams and you'll find that "turtle 
runner" doesn't even appear in the graph: "Ngrams not found". This means the "slow runner":"turtle runner" ratio in English is at most 
1:600, but as it doesn't appear any single year, it's likely much much more skewed towards "slow runner". 
Still, it's not the fact that the expression is NEVER used in English, I found this for example on twitter: 
https://twitter.com/DanielleONeil_/status/1324518213468770305
"Coming from a turtle runner - that pace is amazing!!" Nov 6, 2020 
↑ this English usage does look like it might a "on-the-fly creation" but I think the relative frequency of the construction in Japanese is 
evidence it is at least something of an established expression. I don't think mere coincidence can account for a 1:3 vs ＞1:600 
discrepancy.

Is this an established term or more of an on-the-fly creation? The low number of hits on Google and Twitter suggests to me that it's the latter, and is therefore not needed as an entry. I can imagine someone saying "亀ランナー" without ever having heard it before.

亀ランナー	232
friend's social media post on taking up 
running again: 少しずつ走る感覚が戻ってきてる気がす
る。相変わらず亀ランナーだけど…
twitter: 2 tweets past week.
"朝の10キロで久しぶりに月間100キロ達成。ようやっとる。
自分で積み上げたものしか信じられないと思ってるので(チャ
ントをパクるな)、ヘタレ亀ランナーらしくコツコツ積み上げ
ていきます。"
"私は亀ランナーなので
区間賞なんて全く狙ってません"
"わしが1番スピードから離れてしまってる亀ランナーになって
るわ"

Not all that common