As muscles become fatigued, they can compromise the efficiency of a runner's movement and increase the oxygen requirement for running. Your running can start to fall to pieces even though your cardiovascular system may be ready for more. By expending energy to maintain proper form your pace will suffer. Strength training will enable you to hold your form when you are tired. Strong and evenly balanced muscles help posture, which contribute to good running form.
Weight training helps to maintain muscle mass in an aging person.
More muscle mass equates to a faster metabolism and thus more calories burned. With aging, people are expected to lose muscle mass and gain body fat. Weight training has actually been shown to reverse this process by increasing muscular strength and muscle mass in sedentary older adults. It's never too late to start. Studies have shown that weight training is very effective in preventing osteoporosis in women.