The 27-year-old, from Haverhill, is running in his first marathon in a bid to raise £2,000 for St Nicholas Hospice Care.
He has competed in numerous half marathons and triathlons but is yet to complete a 26.2 mile marathon.
The Haverhill Running Club member has established close links with the hospice, which opened its first outreach centre in Haverhill last year, having founded two races that raised £13,000 for the charity in 2014.
“I’ve been able to develop my running over the last couple of years and am pleased with how I did in 2014, but am now ready to take another step,” said Jack.
“It’s a big jump from a half marathon to a marathon but the training is going well and I’m looking forward to London.
“I’m hoping to use this as a springboard to develop further and ultimately move up to an Ironman-length triathlon.
“The hospice is a great local charity that supports people in Haverhill, and it celebrated its 30th anniversary last year by bringing its first outreach centre to Haverhill.
“The number of people who competed in my events last year showed the level of support for this charity and I hope that support will continue and people will be generous in their sponsorship of me.”
Jenny Baskett, St Nicholas Hospice Care Events and Challenges Fundraiser, said: “We are delighted Jack is running the London Marathon in aid of the hospice.
“We wish him all the best with his training, on the day and with his fundraising efforts.
“The money he raises will help the Hospice to support people facing long term and life-threatening illnesses and their families.”
You can sponsor Jack at www.justgiving.com/JackTappin.