Translation:How many kilometers did you walk?
You use 何【なに、なん】which means "what" (or "how many") + the type of thing you're asking about to ask "how many of that thing".
That's not specific to distances. Duolingo has used 何人【なん・にん】before (that means "how many people").
何歳【なん・さい】how many years old 何匹【なん・びき】how many small animals 何台【なん・だい】how many large machines 何個【なん・こ】how many small round-ish things … And so on
Indeed, we use the imperial system, mostly, though we do also use cm and mm sometimes. Not often, though. I understand that pretty much the rest of the world uses metric, and I like the idea of everything being nice and orderly (everything's divided into 10's in metric but I think 7's or something in imperial?) but I'm not great with measurements already; not looking forward to trying to learn an entirely new measuring system. ._.
歩く 【ある・く】 walk 歩ける【ある・ける】can walk
That's a general pattern with all verbs. For example: 話す 【はな・す】 speak 話せる【はな・せる】can speak
Duolingo has used that word before in an example sentence, by the way.
母は韓国語が話せます。 Mom CAN SPEAK Korean (actually, probably closer to "Regarding mom, Korean can be spoken" because 韓国語 is the subject of the sentence, since it's marked by が)
If we wanted to say "mom SPEAKS Korean", that'd be: 母は韓国語を話します。
Just to be sure since I am not a native english speaker... I do know that 'Kilo' can be used as an abbreviation for 'Kilogramm'. We do the same over here. But does the English language also use it as an abbreviation for 'Kilometers'? Do the Japanese commonly use that as an short-cut for both?
TL;DR: kilo is used in English speech almost exclusively for weight and in Japanese for both weight and distance
First up, I'm also not a native English speaker, however while "kilo" in any scientific context is always a thousand (or close to; see exception in computing like 1 kilobyte being 1024 bytes) of the unit following, like kHz (kilohertz), kg (kilogram), km (kilometre) etc., in speech it is almost exclusively used for weight and not for any other measurement. Whenever it is supposed to refer to something else than weight one would have to specify the unit further in English.
In Japanese this is one of those words that do have English origin (I mean technically it's Greek.. and I also read they have it from French.. so let's just say non-Japanese origin), but the usage within the Japanese language is different. キロ is not only used for weight, but also as a counter for distance in normal speech. Like so many things this will depend on the context it is used in of course. Whenever it is unclear キログラム or キロメタル will be used respectively (or as any other suffix).
There are many loan words that get used in Japanese differently than in their original language. Like the meaning of アルバイト is restricted to "part-time job" or "side job" in Japanese, while in German "Arbeit" refers to any type of work, especially full-time.
Hey, funny thing: You know how miles, yards, feet and inches are defined?
Well, all boil down to inches and the inch is defined to be exactly 2.54cm = 0.0254m, while the meter is defined as the distance light (in vacuum) travels in 1/299,792,458 seconds.
Laughs in physics ;)