As always, there's a relevant xkcd, but this time, it's a What If?
The question is asked,
If you suddenly began rising steadily at one foot per second, how exactly would you die? Would you freeze or suffocate first? Or something else?
Of course, this supposes rising at a steady rate, which may or may not have been the case with aunt Marge, who may have risen like a balloon, slowing her ascent, until she floated at a certain height.
Actually, hopefully she did, as we'll see shortly.
As Randall notes, it gets cold up there, but not too cold:
If you were nude, you'd probably succumb to hypothermia somewhere around the five hour mark, before your oxygen ran out. If you were bundled up, you may be frostbitten, but you would probably survive ...
... long enough to reach the Death Zone.†
At 8,000 meters, the oxygen content in the air is so low, your lungs no longer take oxygen from the air — they lose it to the air. You suffocate.
†: Not related to the Twilight Zone. It's not a journey into a wondrous land of imagination. You're not moving into a land of both shadow and substance, of things and ideas. You just die.
If we accept Legion600's rate of ascent of 1 meter per second, she'd be dead in 8000 seconds and a bit — two hours and a half, at the most.
If we accept Pureferret's time to reach Sheffield, alive, it means she hadn't reached the Death Zone yet. This puts her rate of ascend (again assuming a constant rate) much lower than 1 meter per second, lower than a foot per second even. To travel 8 kilometers in 18 hours, you would have been traveling at the wonderfully magical sounding rate of 0.12345679 meters per second.
Unfortunately, this would put her in the cold zone much longer, so she would've frozen to death after all.
Going back to Randall's article, we see that the temperature is low, but bearable at about 2000 meters (roughly 6500 feet). To reach that height while ascending in a steady rate, while still traveling 10 miles per hour horizontally, she wouldn't have been rising, she would have been going off at an angle, almost horizontally even.
So we in conclusion, she probably rose pretty much like a balloon, quickly at first, but steadily slowing her ascend until she reached a height of about 2000 meters at the most, where she floated, drifting to Sheffield.