"It is tiring to run a lot."
Somebody more fluent than me is free to correct me if I'm wrong, but I think a more literal translation of Duo's Japanese here (「たくさん走るのは疲れます。」) would be "As for running a lot, I get tired." This is because つかれます actually functions as a verb in the given sentence, and its subject should be inferred as "I" by default. (Remember, は is not a "subject" particle, and "Running a lot gets tired." doesn't make sense.)
I think a Japanese sentence closer to "It is tiring to run a lot." would be「たくさん走るのは疲れることです。」(literally: "As for running a lot, it's a tiring thing." or "Running a lot is a tiring thing.").
As I understand, there is no default subject. Duolingo tends to put "I" because a subject is needed in English. Other subjects are usually also accepted. While I am not certain the construction is appropriate (I would need to ask my sister, who has lived in Japan longer than most Duolingo users have been alive), I might think of this as "As for running a lot, it gets tired."
You need to make a nominal group for the は particle, hence the use of の. I've read somewhere that it used to not be the case in old Japanese, but I don't know much about that. In any case it is safe to assume your first sentence will sound wrong or at least to a native speaker (I just asked a japanese friend to confirm).
Also, I guess it's a typo but 疲れます,not 疲れです