Training for and completing a marathon improves the health of a new runner's arteries, cutting about four years off their "vascular age", .
Researchers from Barts and University College London tested 138 novice runners attempting the London Marathon.
Over six months of training, their arteries regained some youthful elasticity, which should reduce the risk of heart attacks and strokes.
And their blood pressure fell as much as if they had been prescribed pills.
Those who were the least fit beforehand appeared to benefit the most.
And smaller amounts of aerobic exercise are likely to have a similar effect, according to the British Heart Foundation (BHF), which funded the study, in the Journal of the American College of Cardiology.
How fast did they run?
It took them between four and a half and five and a half hours, on average, to run the 26.2 miles.
Is it dangerous?
Runners with a pre-existing but undiagnosed heart condition have died attempting marathons - but this is very rare.
Lead researcher Dr Charlotte Manisty said: "People with known heart disease or other medical conditions should speak to their doctor first.
"But for most people, the benefits of taking up exercise far outweigh any risk."
BHF's Prof Metin Avkiran said: "The benefits of exercise are undeniable.
"Keeping active reduces your risk of having a heart attack or stroke and cuts your chances of an early death.
"As the old mantra goes, 'If exercise were a pill, it would be hailed as a wonder drug.'"