system
Given the following post, respond as if you're commenting on Reddit:
human
[Skyrim] How is Astrid able to transport my over-encumbered, heavy armor wearing badass Dragonborn from the heavily guarded Fort Dawnguard to an abandoned shack in the middle of nowhere in such a short period of time?
gpt
Astrid leads a band of assassins trained in various forms of infiltration.  They have their mission, move the Dragonborn to the abandoned shack.  Easy.

Firstly, they scope out where he's going to be at his weakest.  He favors Fort Dawnguard for rest and recovery.  Astrid orders Babette to create a haven of vampires out in the Reach.  She then uses her network of couriers to relay to the Dawnguard that this haven is planning on attacking Markarth in force.  The majority of the Dawnguard forces leave to attack the haven.  The Dawnguard leave Agmaer in charge of holding the fort (a job he'll fail, his inexperience leaves him vulnerable to being incapacitated)

The first thing Astrid does is place a paralysis spell on the Dragonborn while he is asleep.  She gets a recruit to prepare a carriage.  Having access to advanced alchemies, she prepares a few potions of Fortify Carry Weight, feeding one to the dragonborn.  The more brawny Dark Brotherhood members carry his body out to the carriage.

The Carriage is drawn by none other than Shadowmere, who normally wouldn't be caught dead in this position but understands the importance of this job.  Along the way, Astrid maintains the spell and potions effects, before finally arriving at the abandoned shack, where she then puts the Dragonborn to her test.

*Or, you could suspend your disbelief and accept that you didn't join at the intended time of the developers*