I was so immersed in work and hobbies (during my 20s and 30s) that I missed the chance to get married
During my 20s and 30s I was so engrossed in work and hobbies that the chance to get married slipped by
During my 20s and 30s I was immersed in my work and hobbies and before I knew it I was almost 40 and single.
「仕事や趣味に没頭しすぎて婚期を逃した」＝
I was so immersed in work and hobbies (during my 20s and 30s) that I missed the chance to get married
During my 20s and 30s I was so engrossed in work and hobbies that the chance to get married slipped by
「２０代後半～３０代前半まで仕事や趣味に没頭してたらあっという間にアラフォー独身でした」＝
During my 20s and 30s I was immersed in my work and hobbies and before I knew it I was almost 40 and single.
ボキャブラリー
没頭する ＝ be immersed in / be engrossed in
婚期 ＝ chance to get married
逃す ＝ miss (the chance to get married) / slip by
あっという間に ＝ before I knew it
アラフォー独身 ＝ almost 40 and single
イギリス 
