A lot of people ask how long should I train for and what exercises should I do. Well, this depends on your goal. If you want to run a marathon, then swimming is not going to help build the muscles required to complete the task. You would be best to start a marathon training program that focuses on building speed, endurance and recovery. If your goal is to be a bodybuilder, then you will need a program that focuses on building muscle size, rather than just building strength. If it’s weight loss, then you need to be in a calorie deficit (i.e. burn more calories than you consume) so you would focus more on cardio as you’ll burn more calories with a cardio exercise than with strength training (1).
However, for general health and fitness, the American College of Sports Medicine (ACSM) and the Centre for Disease Control and Prevention state “Every adult should accumulate 30 minutes or more of moderate intensity physical activity on most, preferably all, days of the week” (2). Some examples of moderate intensity activities include brisk walking at 3.0 – 3.4 mph/4.8 to 5.5 km/h, cycling less than 10 mph/16 km/h or light to moderate home exercise (3).
Some key points, if your goal is for continuous improvement, then it is important that your workouts incorporate progressive overload – for example, to build strength, you will need to gradually increase the weights that you use. If you want to be a better runner, then you will need to gradually increase the speed, duration or distance that you run.
It is also important that you incorporate rest so that your muscles can recover. The general recommendation is 24 to 48 hours of rest before training the same muscle groups. So, if you did arms on Tuesday, you may want to wait until Thursday to train them again. Recovery is essential to allow your muscles to repair and grow stronger.
(1) Sarnataro, B. Exercise to Lose Weight. WebMD. https://www.webmd.com/fitness-exercise/features/exercise-lose-weight#1
(2) Pate, R., Pratt, M., Blair, S. et al. Physical activity and public health. A recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA 273:402–407, 1995.
(3) The Muscle Mechanic. What are Metabolic Equivalents (METs)?. https://themusclemechanicuk.com/what-are-metabolic-equivalents-mets/