Ben Tench, 48, will run seven marathon distances in seven days to the top of major mountains in England, Scotland and Wales.
Not only will he be running the marathon distances each day, he will be carrying an army style backpack weighing 80 pounds.
The charities which will benefit will b Gone Tabbing, Pilgrim Bandits, SSAFA The Armed Forces Charity, the Curtis Palmer program and the RMA Royal Marines Charity.
Ben, 46, will tackle Ben Nevis, Scafell Pike, The Yorkshire Three Peaks, Cadair Idris and Shropshire's Long Mynd, finishing off on the Brecon Beacons.
He was part of the Royal Logisitics Corps Reserves in Telford for seven years and works outdoors in events erecting temporary buildings and considers himself naturally fit but said it is one of the toughest things he will have attempted to do.
Ben said: "This will be the most demanding challenge physically and mentally I will have ever attempted and I have done a few endurance events in the past,
"I have been in training for 12 months for it and try to do regular 15-30 mile runs but to do seven marathon distances in seven days will be quite daunting.
"All of the charities I am running for do fantastic work for injured veterans and serving armed forces and blue light personnel both physically and mentally along with supporting their families.
The challenge will take place from October 9 to October 15 and a justgiving page has been set up to support Ben's causes.
To support him visit www.justgiving.com/crowdfunding/bentench-7marathons7days80lb-onthemountains.