Knowledgeable competitive athletes plan their training programs months in advance, using a technique called background and peaking. First they spend many months in background training, in which they work out for long hours, mostly at low intensity, followed by a shorter period of peaking training in which they do far less work, but at a much greater intensity.
A distance runner may run 100 miles a week during his winter background training. A few months before her most important racing season, she reduces her workload to around 40 miles a week, but she runs almost as fast as she can two or three times a week.
In his background period, a weight lifter lifts many tons of lighter weights. As he gets closer to his main competitive season, he takes workouts in which he lifts very heavy weights, but does far fewer repetitions. In his background period, a shot putter lifts tons of lighter weights each week and throws mostly for form, not distance. Then as he gets into his season, he does far less lifting, but with heavier weights. He also spends one day a week throwing as far as he can. You can do the same. Start your exercise program at low intensity and low volume. Gradually increase your workload for several months before you try to run fast, lift heavy or exercise intensely.