"It runs for seconds."
Translation:Es läuft Sekunden lang.
firstly, you used 'er' not 'es'; rennt sounds to me appropriate only if whatever 'es' is, has legs. Laufen is the verb which is used in multiple contexts as 'go' is in English. Check here: https://dict.leo.org/englisch-deutsch/rennen and here: https://dict.leo.org/englisch-deutsch/laufen. Or https://forum.duolingo.com/comment/3504131
As an English speaker I find it very easy to get confused, German is very specific in word choice, one could not use verpassen and vermissen interchangeably even though the English uses the same word. You could not use rennen for 'das Wasser läuft über' either. It is something one must learn. https://german.stackexchange.com/questions/18395/is-there-any-difference-between-laufen-and-rennen