When trying to complete a workout, you should try to follow the FITT Principles;

    The Basic FITT Principles:
    How often you exercise every week
    Three 30-minute aerobic exercise workouts is the bare minimum exercise frequency to achieve benefits.
    It's okay and preferable to perform aerobic/cardio workouts more than 3 times per week, but there is a limit.
    If you exercise too frequently you may cause your body to overtrain, which will slow your results.
    Overtraining syndrome can be a serious problem for the small minority of people who exercise excessively. 
    The amount of work you put into each workout
    Your average heart rate, or how high you maintain your heart rate during your continuous aerobic exercise is your intensity.
    The definition of resistance training intensity is how heavy each set is in relation to your maximal lift.
    It is important to know the textbook definition of intensity, because it is easily measured and should be controlled to
    progress your workout routines.
    If you do not keep track of your workout intensity with cardio or weight training you will not know if you are
    putting your body through the overload principle. Increasing cardiovascular and muscular stress above normal
    levels to force the body to progress and adapt is the overload principle in a nutshell.
    How long your workout routines are
    There is an inverse relationship between workout time and intensity. The higher the intensity,
    the less time you can work out for and vice versa.
    With cardiovascular training there are two primary ways to exercise. High intensity training is shorter
    duration training usually in intervals such as wind sprints or any cardio machines interval programs is one.
    Low intensity, low duration training such as cross country involves a slower pace in which a relatively low heart rate
    can be maintained for the duration of the exercise.
    With resistance training, time is slightly more difficult concept. Since resistance training usually involves periods
    of work (sets) and rest, the time on the clock is not as important a factor. How long your muscles are under
    tension is the try time measure for resistance/weight training. 
    For form of activity you perform for each workout
    What is your favorite and least favorite form of exercise? Type of training can affect frequency, intensity as well as time.
    Types of cardio training include; walking, running, hiking, climbing, bicycling, sports and anything which can
    get your heart rate up into and past your target heart
    Your fitness goals will be important for determining the type of weight training workout routines to use. If you are
    looking for strength you should focus on higher intensity strength training. Olympic lifting may be preferential for
    your quintessential power or speed strength is a priority.