While it is technically possible to train for a marathon and weightlifting competition at the same time, it is doubtful you will do well at either one, and run the risk of injury secondary to over-training. If your goal is general strength and conditioning, a few weightlifting training sessions a week and cardiovascular exercise on your off days will improve strength, power and health. If you wish to do well at one activity, you need to specialize. If you go for a limit run the day before a weightlifting meet, you are going to be too tired and sore to lift very much. If you snatch and clean heavy the night before a race, you are going to be stiff, sore and slow, so only you can determine how much of each.