Fisk got his hands on a youth-returning elixir.
Which was a good thing considering his 70 year old body was starting to have trouble keeping up with his enemies, especially a certain Devil.
Fisk had carefully prepared the process of his reappearance, both as the new Kingpin and as a new young mayor candidate.
The only thing the now-has-the-body-of-30year-old hadn't anticipated was how, um,
how high libido a young body would be.
Fisk repeatedly caught himself staring at Daredevil's lips, and chest, and plump ass instead of beating the shit out of his enemy.
And from how red the blind man's face was, the vigilante must be realized Fisk's predicament.