흐르다 is flowing — water, a drink down your throat, and above all time. The doubled 흐르고 흐르다 in four of these lines is a storytelling move for years going by: 어느덧 시간이 흐르고 흘러, 'and as time went on and on'. It also chains plainly, as in 목구멍을 타고 흐르는 15도의 뜨거움, warmth running down the throat.