"I am not walking any more today."
Translation:Ma már nem sétálok.
Maybe because of the accentuation. "Sétálok" should be used instead of "sétalók". Sétalók is not a proper word in Hungarian. The other possible explanation is that "Nem sétálok ma már" sounds a little bit unnatural. Different word order conveys different message in HU. By default, the verb carries the core message; any information that is more important will precede the verb. The most important information here is "ma már".
This is related to objects (in grammatical sense). This is an overly simplified explanation, but think about it like this: When you say "Ma nem sétálok", it means that "I don't go for a walk today". This is simple, there is no object in this sentence.
When you say "Ma nem sétálok többet", it means "I don't walk today more (than I walked so far)". In this case, we need an object such as kilometres (Ma már nem sétálok több kilométert – I don't walk for more kilometres today) or time (Ma már nem sétálok több órát – I don't walk for more hours today). The "-et" ending replaces this object to make the sentence simpler.
The difference between "mi" and "mit" is similar. "Mi" refers to the subject of the sentence, while "mit" refers to the object of the sentence.
In general, when "-t" (which is usually supplemented with a vowel if the word ends with a consonant) is appended to a nominal (like a noun, numeral, adjective, or pronoun), then it refers to the object of the sentence.
Compare these two sentences: A könyv unalmas. - The book is boring. (In this case "könyv" is the subject of the sentence.) Szeretem ezt a könyvet. - I like this book. (In this case "könyvet" is the object of sentence. Note: "ezt" [this] is not the object of the sentence. In this case it just specifies, which book you are talking about. It has to be matched with the object, though, that's why "ez" got a "-t" ending.)
It means "I don't walk again today", but the word order is not correct. Grammatically, "ma nem sétálok újra" is correct, though it's sounds a little bit weird. More natural would be "ma már nem sétálok újra" or "ma már nem sétálok megint". Although, in my opinion, the best solution would be "ma már nem sétálok még egyszer" (I don't walk once again today).
I tried "Ma én nem sétálok mar", and had it rejected. My reasoning was that "Ma" should be the topic, "én" should be included since the sentence seems to be very much about me, "mar" being an adverb of sorts should go with the verb, but be pushed after it by the "nem" taking the preverbial/adverbial position.