Translation:They usually go running or hiking on Sundays.
When Chinese say 爬山 99% of the time they mean hiking or just a walk with a small hill!
I can remember having a confused conversation (雞同鴨講） with an elderly Chinese woman once, who insisted she went "mountain climbing" every week. It turned out she meant a Sunday stroll in the park…文化不同常常會搞不太情趣 …
Well, male or female, at least (as it also applies to children, not just to adults), but I believe the question was about when we hear the sentence out of context, as Duolingo was marking only one pronoun correct.
There's simply no way to know without context, of course, and it doesn't really matter with spoken Chinese anyway, because the pronunciation is the same.
(Incidentally, the feminine "她" in writing is a relatively recent phenomenon in Chinese, and is influenced by Western literature.)