This is set c. 302AC, after all the requisite wars. Everybody lives. Part of the bridge crossing deal was Arya to marry a Frey "a year and a day" from her flowering, remember? Jon's parentage remains unknown.
Arya woke up to a wet, sticky and smelly bed. And her tummy hurt.
She had flowered.
Arya knew what will happen now. Mother clucking on how she is now a woman grown with a side dish of Duty. And either avoiding her or getting clingy, she was not sure which was worse. Robb will look grim, talk of Duty and Honour (the rotter!) and get drunk. And avoid her. Bran will just look sad. Rickon was still too young to be enlightened on female fertility related issues so he would only be informed that a Good Thing had happened to her and that "Arya will be leaving us in a year's time". Wincing from a cramp she thanked the Old Gods that she did have family which actually cared.
Over a year ago he had told her that he had arranged everything for her "when the time comes". Arya believed him. Also she had nobody else left to trust. And she was SO NOT marrying some Frey ratboy ...
Arya waited for appropriate weather. When a drizzle descended on Winterfell she claimed exhaustion (yay for flowering!) and went to bed early. And then slipped out from the castle mingling with the last batch of smallfolk leaving the grounds before the closing of the gates for the night. She headed for Wintertown. Being well aquatinted with the town Arya knew where to go and knocked at the back entrance of a newly established merchant. Once a servant came she demanded to see the owner.
The man – whom she knew to be Waldorf - looked to be in his mid twenties. Arya was fairly sure that she had seen him before. He bade the teenage servant to leave – and who left with a knowing smile on her lips and visibly on the verge of a giggle (silly snot! Is sheathing swords all she thinks about!?) - and looked at Arya expectantly. She bade him stoop to her level (why the fuck was she so short!) and hissed into his ear:
After a moment the man gave an equally whispered reply:
"Is the lady leaving now or in the morn?"
The man (not bad looking) took her arm and led her inside the house. A few moments later, in a locked, well kept although evidently unused room the merchant bent the knee – apparently making up for his lack of courtesies when in front of the serving girl - and whispered:
"My Lady. Everything is prepared and ready. All was bought a year ago and has been waiting. Clothing, horses and all. Even the shears. His lordship was generous."
She examined the traveller's supplies – fireboxes, flints, whet stones, etc. The boys' clothes came in a broad selection of sizes – bless his heart. Bow, with spare drawstrings, arrows. Even a hauberk with coif...
Arya rode out in the drizzle hanging in the air before dawn. She had two horses, both saddled and with provisions – we buys them fresh every week, Waldorf had said – split equally between the two. The horses were of reasonable quality and – if she switched her mounts – no pursuit party from Winterfell could catch her (eat my dust!). Before setting out east, towards the Kingsroad, she warged to check where Nymeria was. The wolf was coming.
Two weeks later
Led to one another by their wolves the two embraced. After the first hugs and kisses Jon drew away to examine his little sister – thin and worn by the trek but nonetheless none the worse for wear. He ruffled her short bucket-cut hair.
"Waldorf came through, I see."
"Yes. He even offered to set something on fire as a diversion. He said that you had saved his life so many times during the war that he'd cut his balls off to make you happy!".
"You left the note?"
"Of course, stupid!"
"So what now?".
"I will keep on sending ravens to Winterfell saying that search parties had not found any trace of you - neither dead nor alive. You will live in a hunting lodge in the most remote part of my estate for now. Sadly too many of my staff know you from Winterfell and - loyal to me or not – there are too many too keep a secret. In a few weeks, when things cool down, we will meet again and decide what to do further."
"How many know?"
"Only me and Walda. I had to tell her. She still half expects me to present her with a Snow at any moment, so my trips to the lodge would only make her see what she wishes to see. You know what the place she grew up at was like. I have no intention to torment her."
Arya nodded. Their father's refusal to speak of Jon's mother had been a bone of contention between her parents. Thus her limited experience was that the fewer secrets between spouses the better. And Walda was nice enough. As the fat girl made Jon happy this made her at the very least tolerable in Arya's eyes, Frey or not. Plus the cookies she baked were to kill for.
The two rode towards the lodge making idle chatter. At some point Arya said that Waldorf had been easy on the eyes, had so far proved honourable enough, so if a man like him was on offer she might be open to considering marriage. This made Jon chortle. And after a moment Jon was simply braying in laughter. He kept on looking at Arya and trying to compose himself but failing and bursting out in laughter again. Tears ran down his cheeks as he looked at his sister.
"What? What?" Arya kept asking and hitting his arm.
After regaining capacity for speech Jon rasped out:
"Waldorf Rivers is old Walder's grandson ..."