I know that this is not how Eleven's regeneration actually happen. This was written before the Christmas special. Anyway, perhaps you can consider that it took place in an AU or at least just ignore the first little bit that's not cannon ;)
He told her to run. He could feel the intense heat rise from his core. It spread throughout his body and within a matter of seconds he began to see the golden glow oozing from his skin. The pain was excruciating. It was happening.
A blinding white light overtook him. With a fierce yell he fell to his knees. Holding his head in his hands, he attempted to shield himself from the light and the burning that raged within him.
Suddenly all fell silent. He heard birds chirping in the distance and a gentle wind tickled at his neck. Slowly he began to open his eyes, yet they were still adjusting to the soft lighting of his surroundings. He placed his hands beside his folded knees and felt soft grass between his fingers. Strange, considering that he had just been exposed to winter, just a moment before. Bewilderment struck him as he realized that his hands had not changed their appearance.
Feeling vulnerable, he let out a cry of "What's happened? Where am I!?" just as his eyes darted upon the sights of a summer garden.
A calm voice trickled into his ear from somewhere behind him: "You are an echo, Doctor."
A tender hand arrived upon his left shoulder as the owner of the voice circled around him, moving to the front of his kneeling body.
A familiar smell entered his nose as his eyes were met by a flowing white dress. He slowly gazed upwards following the curvaceous figure until he reached a comforting face. She was more beautiful than he had ever remembered her to be. He stared hopefully into her eyes as his mind raced through thousands of unanswered questions.
"You came back." she breathed, holding back the emotion. She shook her head in dismay as her curls swayed with the gesture.
"River!" he exclaimed out of desperation, "What is going on? Why are you here and I-I'm still me!" His brow furrowed. This was something of which he had no understanding, and that truly terrified him.
"You've regenerated, Sweetie", said River with sincere compassion. "You live on but this part of you, this body and this mind; they…they've died. And here you are. You've found me. You've come back, like you promised. I just wish that it hadn't been so soon."
As she spoke, the Doctor remembered his words and his vow to see her around before he had jumped into his own time-stream. "And you, River? What is this place?" He choked.
"This is the River you met in the Library, dear. As of now, we know everything about each other. And this place, well, if there is such a thing as a heaven, then I suppose this is the closest thing to it. Here we do and do not exist at the same time. We are eternal." River spoke in such a way to try and ease the mind of the timelord.
"And what of Clara? Is she hurt? If she was too close when I…then-" he stammered.
"You have done your best to protect her thus far, Doctor and I'm confident that your latest incarnation will keep Clara just as safe as you did."
The Doctor stared up into River's eyes, tears forming in his, "So that's it then. No more adventures. No more TARDIS. No more seeing the faces of all the people in the universe. That's it. My time is up. I am actually dead." His body began to tremble as his gaze broke away from hers. He stared open mouthed in no particular direction. His mind was tired and saddened.
Unable to bear the grief of her husband, River grasped hold of the Doctor. One hand caressed his back while the other cradled his head. She held him safely within her embrace, trying to ease his pain.
The Doctor's arms reached upwards from his sides and gripped his wife around her back. He held on tight, absorbing the security she presented. He buried his head in her dress and began to sob lightly. River felt the warmth of his tears as they seeped through her gown and dampened her skin. A tear fell from her eye as she peered down at the man she held in her arms. She began to sway back and forth, gently rocking her Doctor.
River continued to hold him as they both tried to come to terms with the events which occurred.
Both had been alone for far too long and now, though not physically alive, their entities could share a life of peace together that they both so desperately deserved.