Hallissey was confirmed as a member of the Great Britain squad for the 2012 Summer Olympics in the women's marathon alongside compatriots Paula Radcliffe and Mara Yamauchi.

[3] She made a gradual transition to longer-distance races, and finished in 24th overall at the 2009 IAAF World Half Marathon Championships.

In 2010, she moved with her husband to Arlington, Virginia, and went on to win several road races in the DC metropolitan area.

[4] She was still able to run internationally in the UK national singlet, as she qualified for the 2010 IAAF World Half Marathon Championships and placed 18th overall.

[3] In 2014, she raced in the DCRRC singlet when she won the 2014 Maryland-D.C. Road Runners 10-Mile Club Championship.