Story highlights Erika Brannock is the last of the more than 250 victims released

She suffered severe bone and tissue damage

"I will use what I've learned to pay it forward," Brannock says

She plans to finish a master's degree and return to teaching in the fall

For Erika Brannock, Monday was a long time coming -- 50 days in fact.

That's how long she was hospitalized after bomb blasts at the Boston Marathon ripped apart her legs.

She was the last of the more than 250 victims from that attack to be released.

"I leave here today -- after 11 surgeries, some pretty dark moments, and 50 days in this hospital -- with nothing but admiration for this great city," said Brannock, who was treated at Beth Israel Deaconess Medical Center in Boston.

On the day of the marathon, she, her sister and her brother-in-law had gone to watch Brannock's mother run.

They were standing near the finish line when the bombs went off.

JUST WATCHED Help Boston victim find her rescuer Replay More Videos ... MUST WATCH Help Boston victim find her rescuer 05:30

JUST WATCHED Tsarnaevs: Gym to marathon in 3 days Replay More Videos ... MUST WATCH Tsarnaevs: Gym to marathon in 3 days 00:10

JUST WATCHED Music's best come together for Boston Replay More Videos ... MUST WATCH Music's best come together for Boston 03:08

"I fell backwards, and I could see oranges and yellows," Brannock told CNN's AC360 on Monday. "I could hear the sirens and people crying and screaming. But I never heard the actual boom."

"I had a conversation with God in my head, and I told him I wasn't ready to go."

Just at that moment, a woman crawled over to Brannock and grabbed her hand. She used her belt as a tourniquet on Brannock's leg.

"She had heard me screaming for help and she said, 'My name is Joan from California, and I'm not going to let you go.' And she stayed with me the whole time."

Brannock, 29, credits this mystery woman with saving her life. She desperately wants to find and thank her.

She also praises her medical team.

Brannock, a preschool teacher from Maryland, suffered severe bone and tissue damage, requiring the amputation of her lower left leg.

"I would not be here today without the talent and devotion of my care team, as well as the first responders and the marathon spectator -- who I only know as 'Joan from California' -- wherever you are -- you saved my life," she said.

'We will thrive again'

The double bombings near the finish line of the Boston Marathon on April 15 killed three people and injured at least 264.

Brannock's sister, Nicole Gross, had a broken leg and remained hospitalized for 33 days.

Her sister's husband suffered cuts, bruises and burns.

"Although we had three family members injured, our family remains intact. We not only survived, but we will thrive again, even though our lives have been changed forever," said Brannock.

The road to recovery is long and sometimes difficult.

She kept a picture above her hospital bed of a dragonfly, which she said was a symbol of strength, courage and getting through hard times. She described the creature as her mascot.

Brannock started having nightmares after she learned that surviving bombing suspect Dzhokhar Tsarnaev was at the same hospital as her.

She dreamed he was going to blow up Beth Israel.

Back in Maryland, Brannock will start physical therapy and will have to learn how to walk with a prosthetic leg. But she says she is up to the task.

"I can get through anything," Brannock said.

Nurses and doctors lined the hallway Monday as she left the hospital, cheering and clapping. She gave a high five to one.

Next on deck? Brannock plans to finish a master's degree in early childhood education and return to teaching in the fall.

"I will use what I've learned to pay it forward," she said.

"Thank you Boston. Thank you everyone. I'm ready to go home."