The urge to run setting in is the first odd sensation to wash over you. An intense need fills your heart: you need to tear through these fields with reckless abandon, and you don't want anything to be blocking the feeling of the cool grass beneath your feet, nor the breeze upon your skin. There's nobody around, and thus you don't feel any hesitation in pulling off your clothes and your shoes, tossing them into a heap before you take off into a sprint. You smile as your eyes shut, letting your mind immerse itself in the feeling of grass trampled beneath you and your personal bits dangling free in the air.
It isn't far into your run before another odd sensation fills your body, though it's still quite pleasant. The weight of your own body seems to hang off your muscles differently - you swear that you can feel your arms and legs becoming lighter, and yet, your chest and your rear end seem to feel heavier. You don't really question it, at least, not until the sensation of your rod hanging in the air begins to fade in tune with your hair tickling at your neck, then your shoulders, then your back. You finally open your eyes to find that you're nearing a small lake, and stop to bend over and get a good look at your reflection.
The image that stares back at you is one you instinctively recognize as your own visage, different though it may now be. Your attention is first drawn to your own delicate, beautiful face, large amber eyes staring back in wonderment as you paw at the long mane of wave blonde hair that now cascades down from your head. Dainty hands trace with wonder over your silky-smooth skin, coming to grab handfuls of a now-bountiful bosom that practically begs to be squeezed. For a moment, you're so lost in rubbing the sensivtive endowments that you don't quite notice the gorgeously thick hips that complete your hourglass figure, nor the delicate flower that now sits between your thihgs.
You should be shocked, some part of you knows this - but what use is there in being surprised or remorseful for this change? Your reflection is that of the most beautiful girl you've ever seen. If this is what the mirror world has in store, then you can scarcely imagine wanting to resist. No, the logical thing in your mind is to embrace it - and so, after a few more minutes of exploring the pleasures of your new body, you set off into another run, giggling as you hope that further feeding the urge for speed will reward you with further changes. You're giddy as the sensation of something shifting and warping within your body returns, eyes wide open now as you crane your neck to watch it unfold.
The slender legs carrying you forward begin to swell, their muscle mass growing, only for hairs to begin covering their defitiniton. Golden fur not unlike your own hair is spreading up them, from your increasingly rotund ass all the way down to your feet. You stumble as those feet began to stiffen and harden, constricting back from long bipedal feet to a more round shape. The feeling of life flesh becoming hard hoof is exquisite, and you can't help but play with yourself, hands reaching deep within your soaking new folds as if the arousal might accelerate the changes. Despite the fact that you're now laying on the ground, chest heaving against the grass, the transformation is continuing unabated.
Your impossibly plump rear end is growing out, becoming long and thick behind you, covered in that same luxurious golden fur. You realize the purpose behind this as it reaches its final length, and two protrusions start to form. Slowly but surely, your new back legs push their way out, front hooves stomping with glee as the weight of your altered body begins to balance itself out. Your first orgasm as a woman thunders across your body when you first rise up in a quadrepedal stance, tall and proud as any golden-furred centaur ought to. A tail swishes with ecstacy as you let out a thunderous moan, so lost in a wave of pleasaure that the idea of going back is nowhere in your mind.
Your equine body is rippling with power that begs to be utilized, and you oblige, breaking out into a gallop across the grassy plain. You're moving faster than you ever could have imagined moving under your own power, the sheer strength and endurance of your lower body carrying you forward at startling velocities. You laugh with glee at the wind rushing past, and to your excitement, you realize there's one last set of tingles along the back of your equine body. A pair of new limbs is spreading out, but it isn't a pair of wings this time. These new extremeities are wide and flat, the skeletal framework of them giving you a hint as to their purpose before the golden feathers emerge.
The idea of having wings had never occurred to you, but as your new pegasus wings stretch out, flapping them to lift off feels as natural as walking. Your speed increases dramatically as you're freed from your earthly bonds, propelled up into the air, soaring high into clouds. You've never felt such freedom, such power - god, how could you ever go back from this? There's more joy in your heart right at this moment than you've felt over the whole of your old life. You make a resolution right then and there: even if you go home, you want to stay like this. You can't imagine anything more wonderful than the pegasus body you now possess. If that ultimately means you have to give up everything you knew before... you feel like you can live with that.