Multiple recent medical research studies have shown that exercise, even just walking for several hours a week, can prolong life, decrease the risk of 13 types of cancer, and even prolong survival time in patients with advanced colon cancer.
A prospective cohort study published in October 2017 in the American Journal of Preventative Medicine followed 62,178 men (mean age 70.7 years) and 77,077 women (mean age 68.9 years) for 13 years. They found that walking between 2 and 7 hours per week decreased mortality by 20% (HR = 0.74 CI 0.72 to 0.76). 
A prospective cohort study published in BMJ in February 2018 followed 1181 men between 71 and 92 years of age without pre-existing cardiovascular disease for 5 years to determine their level of physical activity. This study showed that even mild physical activity such as gardening or walking the dog, decreased mortality by 17% (HR = 0.83 CI 0.77 to 0.90) and if it accumulates 150 minutes per week the mortality decreases by 41% (HR = 0.59 CI 0.43 to 0.81). 
In an investigation published in JAMA in June 2016 with 1.44 million participants and an average age of 59 years showed that exercise decreases the probability of contracting 13 of 26 types of cancer: adenocarcinoma of the esophagus (HR, 0.58, 95% CI, 0.37-0.89), liver (HR, 0.73, 95% CI, 0.55-0.98), lung (HR, 0.74, 95% CI, 0.71-0.77), kidney (HR, 0.77, 95% CI, 0.70-0.85 ), gastric (HR, 0.78, 95% CI, 0.64-0.95), endometrial (HR, 0.79, 95% CI, 0.68-0.92), myeloid leukemia (HR, 0.80, 95% CI, 0.70-0.92), myeloma ( HR, 0.83, 95% CI, 0.72-0.95), colon (HR, 0.84, 95% CI, 0.77-0.91), head and neck (HR, 0.85, 95% CI, 0.78-0.93), rectal (HR, 0.87 95% CI, 0.80-0.95), bladder (HR, 0.87, 95% CI, 0.82-0.92), and breast (HR, 0.90, 95% CI, 0.87-0.93). 
Surprisingly, even patients who already have colon cancer have benefited from walking. In a study conducted by Harvard University of 992 men with stage III colon cancer, it was shown that 30 minutes of exercise per day decrease mortality compared to similar patients, in 42%. 
How does exercise prolong life, even in cancer patients?
It is theorized that because exercise increases glucose metabolism, and therefore decreases the level of circulating glucose in the blood, this results in a decrease in insulin levels and eventually a decrease in obesity.
High levels of insulin result in a decrease in insulin-like growth factor binding proteins 1 and 2 (IGFBP1 and IGFBP2), resulting in high levels of insulin-like growth factor-1 (IGF-1.). High levels of IGF-1 increase the risk of cancer of the breast and prostate, but also accelerate cell proliferation. and therefore the growth of several types of tumors with IGF-1 receptors. An increase in exercise and decrease in carbohydrate consumption results in a decrease in insulin and IGF-1 and therefore decreases the proliferation of cancer cells. 
In summary, in patients with cancer it has been shown that the benefit may come from the decrease in circulating insulin levels, because insulin helps multiply the tumor cells. 
In addition to decreasing insulin levels, and therefore IGF-1, exercise decreases estrogen levels. 
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