So, I am a bit confused here. I got the sentence "I walk a lot." and answered "πολυ περπατω", which was marked as wrong and the correct answer should be "πεφπατω αρκετα", which seems odd, because πολυ would be the literal translation of "a lot".
Both ''πολύ" and "αρκετά" are correct. I think the problem was that we have only: "περπατώ πολυ' περπατώ αρκετά'' but not "πολύ περπατώ" which of course is also correct and which I've now included. Sorry about that omission and thank you for the input.