Answer by taking a quote from the following article:

Mary Teresa Slaney (formerly Tabb, nee Decker; born August 4, 1958) is a retired American middle-distance runner. During her career, she won gold medals in the 1500 meters and 3000 meters at the 1983 World Championships, and was the world record holder in the mile, 5000 meters and 10,000 meters. In total, she set 17 official and unofficial world records, including being the first woman in history to break 4:20 for the mile. She also set 36 US national records at distances ranging from 800 meters to 10,000 meters, and has held the US record in the mile, 2000 meters and 3000 meters since the early 1980s, while her 1500 meters record stood for 32 years.

In her early teens, Decker was already recognized as a world-class runner. Unable to attend the 1972 Olympics as she was too young, the pigtailed 89 pounds (40 kg) 14-year-old nicknamed "Little Mary Decker," won international acclaim in 1973 with a win in the 800 meters at a US-Soviet meet in Minsk, beating the later Olympic silver medallist.  By the end of 1972, Decker was ranked first in the United States and fourth in the world in the 800 meters. In 1973 she gained her first world record, running an indoor mile in 4:40.1. By 1974, Decker was the world Indoor record holder with 2:02.4 for 880 yards, and 2:01.8 for 800 meters.  By the end of 1974, she had developed a case of the muscle condition compartment syndrome. This resulted in a series of injuries, which meant that she did not compete in the 1976 Olympics, because of stress fractures in her lower leg. In 1978 she had an operation to try to cure compartment syndrome, which kept her out of competition for a period. After recovering from surgery, she spent two seasons at the University of Colorado at Boulder on a track scholarship. In 1979, she became the second American woman (the first was Francie Larrieu) to break the 4:30 mile in American record time. Decker was the first woman to break the 4:20 barrier for the mile in 1980 when she ran it in 4:17.55. However, this time was never ratified by the IAAF. In 1981 she married fellow American distance runner Ron Tabb. The couple divorced in 1983. In 1982, under the name Mary Tabb, she ran the mile in 4:18.08, breaking the official record of 4:20.89 by the Soviet Lyudmila Veselkova This time was ratified. She did not compete for an Olympic medal due to the U.S.-led 1980 Summer Olympics boycott. She did however receive one of 461 Congressional Gold Medals created especially for the spurned athletes.

What did she do in her early career?
"Little Mary Decker," won international acclaim in 1973 with a win in the 800 meters at a US-Soviet meet in Minsk, beating the later Olympic silver medallist.