Translation:Yesterday, I ran 5 kilometers.
I would only want Duo to teach more Kanji if it were to accurately tell you what it sounds like when you click on it. As it is now I have to replay the sentence's audio repeatedly in the hopes that I will catch the proper pronunciation for words that use Kanji. I'm on an Android tablet though. Maybe it's different on PC.
I'm afraid Japan is decidedly metric; even shoe size is simply given in centimeters (as in, how long is a foot? 26 cm for me :v)
America and American culture is somewhat popular/fashionable in Japan though, so the average Japanese person may know that Americans use weird units for things, maybe even recognize what pounds, feet, or miles are for, but I wouldn't expect much more familiarity than that.
The average American probably doesn't know about traditional Japanese units of measurement, like the shaku (尺, just a tiny bit shorter than the imperial foot) or the kan (貫, standardized to 3.75 kilograms). During the imperial era, Japan used their own traditional units, English imperial units, and metric units interchangeably. For a few years after the end of World War II, imperial units were imposed by the occupying American forces, but by the '50s onward, Japan committed themselves to metrication, and today the traditional Japanese units are relegated to a few domestic uses such as carpentry, agriculture, land and floorspace measurements, and some traditional cooking utensils; while the English imperial units are nowhere to be found.
A Japanese person knowledgable of English imperial units would likely be either interested in English/American culture or old enough to remember when Japan used those units as well.
My first thought was yes, but on further inspection, I think it's actually a no. The キロ here is acting as a counter, which means it's already modifying the verb by itself. Also the verb, 走る, is intransitive, like many movement verbs, so を behaves differently from what you might expect.
Yes, people would understand it based on the context. キロ can also be used as an abbreviation for kilogram, or even kilometers/hour, but how often would you hear someone say something like "I'd like two kilometers of chicken breast please" or "The speed limit here is 60 kilograms". In ambiguous situations, people might use the full versions of these words instead.
In Japanese schools, I imagine they are taught the full versions: キロメートル (kilometer), キログラム (kilogram), 時速キロメートル (kilometers per hour). Or perhaps even more likely, they're simply taught the SI abbreviations: km, kg, km/hr.