Mark Allerston, 49, of Wykebeck in Leeds, wants to raise money for charity Cardiac Risk in the Young in memory of Adrian Badon from Horsforth.
Mr Badon discovered his partner Charlotte Watterson was pregnant in April 2014.
He died suddenly two months later in June 2014 and did not live to see the birth of his son Drey in January 2015.
Miss Watterson changed her surname to Badon after his death.
Mr Allerston said Mr Badon, who had moved to the Isle of Man with Charlotte in February 2014, did not display any symptoms of an undiagnosed heart condition.
Mr Allerston said: “He was perfectly healthy and there was no indication there was anything wrong with him.
“He was just a normal guy who was a probably a bit overweight.”
Mr Allerston said he started running in 2016 when he weighed more than 20 stone and suffered from high blood pressure.
He started with short jogs, then park runs and graduated to the Leeds 10k last July and the Leeds 10k Abbey dash last November.
Now he is running 14-miles a week to train for the Leeds Half Marathon and is more than two stone lighter than he was in 2016.
To support Mr Allerston’s fundraising, go to www.justgiving.com/mark-allerston1