Translation:One of the athletes is not running, but walking.
We need to be patient, there is sooooo much to correct in the course. But I have seen much improvement already. So, please, just keep reporting the issues.
On "one of the athletes", you could translate it more closely as "a sportolók egyike". So you have your plural athletes. It is perfectly okay to say it like this, but the other way ("az egyik sportoló") is much more common.
Now it is a year later, and it still has not been corrected. I also am getting the feeling that work has stopped on the Hungarian duolingo. The same problems come up over and over again, I report over and over again, but nothing is happening. I used to get reports saying that my suggested version was good, and is being included. I no longer get those messages. While I appreciate everything that has been done. it is becoming increasingly frustrating.
I find "Az egyik" strange. The English sentence does not identify the athlete in question yet the Hungarian uses the definite article. Is there a shortcoming in the translation? Maybe the English can not be accurately and concisely translated in such instances? I've noted jsiehler's comment but find the English not to be idiomatic.