“Look out for that, oh, boy, right into the propellers!”

“I kiss my frogs in your honor.”

“That salespony told me I could fly with these wings. I want a refund!”

Y’know, pumas aren’t exactly known for flying.

@Zaedrin  
“My horn makes me fall sideways!”

“She has the power to rear-project major cities!”

I didn’t know Twilight was Polish…

“I’ve plummeted to the earth and I can’t get up!”

I kiss my frog in your honour.

“HELP, I’M FALLING AT A SIXTY DEGREE ANGLE BREAKING ALL THE LAWS OF PHYSICS!!”

@Mayojar77  
See previous response.
 
TBH, I don’t see her as being any less fit than the other townsponies’ average. She’s likely to have good exercise habits and diet through having a very privileged upbringing, being well read in health matters, and having had an excellent education. Pretty much like the offspring of the professional classes in many human cultures. While this is an argument set out in averages and bell curves and shit, and we’re talking about just one pony, I think it does make it more plausible that good health aspects of her lifestyle could offset the bad.
 
The Running of the Leaves is presented as a fun run rather than a marathon, and quite a number of the runners seem to be there just for enjoyment. In fact RD and AJ’s hypercompetitiveness relative to the rest of the herd is a major plot point.
 
But yes, I will concede that one episode in the first half of S1 of a show about brightly coloured magical talking ponies has a slightly unbelievable plot point.

@Mayojar77  
Well then, I have no ideas.  
Maybe the writers are just inconsistent.

@Alalic  
She had only decided she was going to join in the night before. She had no interest in the holiday before then. She managed to get a place in a marathon despite the fact that all her preparation consisted of one late-night cram session.

@Alalic  
That’s some RPG-level crap going on there.

@Mayojar77  
Unfortunately, Fall Weather Friends is one of the few episodes that I’ve not seen.
 
However, reading a book isn’t an instantaneous task either. Twilight would have had time to go back and read the book over and over again, like how a kid can teach themselves how to play a guitar using a book. Knowing Twi’s obsession with books, she would have been able to get to a passable level of running skill just by reading.

@when she felt her wings unfold  
Are you completely ignoring the fact that she was able to run a marathon after just reading a book, despite having no previous experience with marathons and living a very sedentary lifestyle since she was in Kindergarten?

@Mayojar77  
Apparently all but four (?) of the background ponies are hypercompetitive fucks too.

@Mayojar77  
Her magical copying ability was explicitly limited in the show to other unicorns’ magical spells. Not other skills, and that would include fancy flying, or even the basics it seems.
 
The ability evidently doesn’t apply to alicorn magic, or Magic Duel would have been a very short episode and she’d’ve been able help Cadance out directly in the S3 opener.
 
My take is that she can copy a spell-based unicorn skill based on a single observation and use it, but won’t have the finesse to get it working just as well as the other pony without putting in just as much practice as they would. So maybe Rarity can find shinier gems faster, and it certainly looks as though Shiny can project a tougher and bigger force-field.

@Alalic  
What about that time she learned how to be an at least adequate runner after just reading a book, despite being easily the least fit of the Mane Six?

@Mayojar77  
All of those relate to Twi’s special skill: Magic. Of course she would be able to cast unfamiliar spells after seeing them a few times; it’s her cutie mark after all.  
Flying using her wings probably does not require Twi actively casting magic, therefore that skill of hers does not apply.

@Alalic  
Do you not remember The Crystal Empire? Twilight only saw Celestia use that spell once, and was never told how it works.  
Or how about that time she used that time-travel spell? And I highly doubt anyone would have intended for Twilight to learn the ‘Want It Need It’ spell.

@PonyPon  
Maybe? IIRC Twi got that spell from a book, so it’s possible that the spell came with some knowledge on flying.
 
@Mayojar77  
What ability? I don’t recall anything like that.

Subby gets a ram chip!

@Alalic  
Twilight has many times already demonstrated the ability to learn things she’s only ever seen done once before. Rainbow Dash spends most of her time at approximately three times Applejack’s height, and that’s only the waking hours.

@Alalic  
On that note, is it weird that Rare’s butterfly wings from way back when didn’t cause so much trouble for her? Do they just come with magical competence or something?