Grandad Tom Carr, from Wellingborough, and Liam Culley are 4.2 miles away from reaching their 26.2-mile landmarks, though Tom says he's got a bit of catching up to do as his grandson has raced a little ahead of him.
"A few years ago my grandpa learnt how to swim and once he had accomplished this he took part in the Swim22 challenge 2017," said Liam, from Wootton.
"He has now completed this twice and this year I was swimming with him when he began the journey once again.
"I decided that him doing the challenge is not enough and I took part myself.
"We began in Roscommon in Ireland and I seem to be doing a lot more laps than grandpa so I'm slowing down on how often I go swimming so that we can hopefully finish it together when we are altogether with my family in on holiday in France.
"We have now completed the original 22 miles but have agreed to carry on and complete 26.2 miles, a swimming marathon."
In total, Liam and Tom will swim 1,416 lengths to raise money for Diabetes UK.
"I took early retirement and I always wanted to learn how to swim - it was on my to-do list," said Tom.
"I've got type-2 diabetes and three years ago I learned to swim and since then have done the challenge every year.
"I took Liam to Ireland and he said to me 'can I do the challenge with you next year?'
"We have swum at separate times but we swim together at weekends."