Translation:It is tiring to run a lot.
In this sentence, の normalises the verb into a noun (makes "run [走る]" into the "activity of running a lot" [たくさん走るの]), while は is the topic marker, making たくさん走るの the topic of this sentence.
Shouldn't "To run a lot is tiring" work? Or is that grammatically incorrect?
Grammatically, it should work, but it isn't a natural sounding sentence. Although there are exceptions, I think English speakers tend to avoid starting sentence with an infinitive (to verb).
Let's be real though, with the amount of "unnatural" translations Duo has (and often enough outright encourages), especially here, I think it would be par for the course. (Pun not intended.) Perhaps rather literary or formal than everyday, but I'd argue it's quite natural, certainly more so than many other examples I've come across in this Japanese course.
As a native English speaker, I concur that this construction is a bit too odd, if not exactly incorrect.
Yet somehow as a native english speaker this is how I chose to answer. Some times Duo likes a more literal translation, some times it prefers the more "natural" sounding phrase.
English generally doesn't start a sentence with an infinitive "to run a lot"
Similarly, I wrote "lots of running is tiring." Would that technically be incorrect?
That's what I wrote, too. It seems to me that this would technically be correct, but I'm still not sure.
I agree - that is also pragmatically the same and therefore correct. This app is really bad at understanding all of the acceptable forms of English.
Is there a way in which つかれます could mean "I am tired"? Duo doesn't accept "I am tired from running a lot" but is it wrong?
Adding on this, the correct sentence of "I am tired from running a lot" is たくさんはしったのでつかれました
I am tired has to be つかれました in past tense in Japanese because it is a state verb.
This is similar to おなかがすきました (I am hungry), right? So I've always wondered how you express the past tense with verbs like that. I guess you could add in きのう or せんげつ but besides that?
Mine was "running a lot makes you tired." That's different from the other answers given here, but is it off in a meaningful way?
No, I think that should be acceptable. Going the other way, つかれますよ feels like a better translation though.