Marathon - this requires endurance, and, therefore, primarily utilizes Type I muscle fibers (slow oxidative). You will have an increase number of mitochondria (need an increase oxidative phosphorylation); [myoglobin] increases to provide muscle with more oxygen needed for oxidative phosphorylation; myocyte numbers never increase since they stay in the G0 phase; there may be an increase in the contractile proteins such as actin and myosin, but to a lesser degree than in weight lifting
Weight lifting - hypertrophy of type II muscle fibers (fast glycolytic); [mitochondria] doesn't really change; less myoglobin than in endurance athletes (white appearance of muscle due to lack of heme pigment in myoglobin); again, myocytes (aka muscle cells) don't increase...hypertrophy (not hyperplasia) is due to an increase in sarcoplasm and contractile elements such as actin and myosin within a myofibril
Sprint - basically the same as weight lifting, except you don't get as much hypertrophy of the muscle