Calling the genders in plural 'masculine' and 'feminine' is oversimplifying. Their names could be rather translated to "masculine personal" and "not masculine-personal". Therefore the only* nouns that are "really" masculine in plural, are groups of men-only and groups of mixed gender. Groups of women, and all the other words, are 'feminine', or rather 'not masculine-personal'.
*Okay, that's another simplification. It also works for personified animals of male gender (like in cartoons) and for something else which I cannot remember nor find right now.
What's the difference between chodzić and iść? Also when can you translate them as 'go' or 'walk'? Finally, why doesn't Duolingo accept 'am/is/are walking/going'?
chodzić = to go, to walk (generally)
iść = to be going, to be walking (right now)
Also, "to be walking" (with no purpose nor direction, just walking around) = "chodzić". So this sentence is one of the rare examples where it is accepted.
The possessive pronoun "jej" just happens to be the same as the genitive of "ona". Third person possessive pronouns don't inflect for case. And moje is nominative here. There's no reason for genitive.