She completed the swim in a total of 19 hours and 55 minutes, which was 10 hours shorter than the previous record.
Literal
She [topic-は] total [in-で] 19 hours 55 minutes [in-で] swam-completely [but-が], this [topic-は] until-then [possessive-の] record [than-より] 10 hours was-short.
泳ぎ切る is a compound verb using the productive suffix ~切る, which adds the meaning 'to do completely / to the very end.' Attached to the masu-stem of 泳ぐ (to swim), it conveys swimming the entire distance without stopping. This suffix attaches to many verbs: 走り切る (run the full distance), 読み切る (read through entirely), 言い切る (state definitively). The second clause compares her time against the previous record using より (than).