Strength Training Comes First.
Exercise recommendations will be unique to every individual, but IN GENERAL when you are training to build a strong body and a healthy metabolism, your primary focus will be on muscle development and strength training. Cardiovascular exercise will come secondary, but this does not mean that the benefits of cardio are not important. When properly working through a well-structured strength training routine, you will find that you are getting in plenty of heart healthy movement from this alone. Even so, cardiovascular exercise should generally be your supplement, with resistance training as the primary modality.
If you are not already on a good strength training program, my recommendation is to set you up with The Weekly Lift on my app, The Fit Foodie. This is a structured strength-training program, for in the gym or at home, for all fitness levels (beginner to advanced) that progresses weekly to help you build strength, develop a toned physique, increase your metabolism and improve your overall health and energy. The workouts are super easy to follow with a very user-friendly layout, including movement tutorials and/or form tips for all exercises.
Everything you need to get started with The Weekly Lift can be found on my app. Click HERE to download. The Weekly Lift is totally customizable, and I will be working through the specifics with you on exercise frequency and intensity depending on your goals.
Cardio Comes Second.
Generally, when your focus is on building muscle you will not want cardiovascular/ aerobic exercise to be the primary stimulus to your body. Though cardio is healthy for your heart, too much cardio - and too high in intensity - will hinder your progress with strength training and building muscle, as well as cause your metabolism to slow down.
The best way to incorporate aerobic exercise in alongside your weightlifting routine is to keep your cardio sessions lower in intensity and in forms that are more easily incorporated into your day. As a general rule of thumb, look to get in some form of cardio 2-3 days per week, about 20-30 minutes each. Again, I will go over anything more specific with you if needed.
Walking & Low-Intensity Cardio:
Walk more/ aim for 10k steps - Moving more throughout the day is a great way to incorporate fat-burning activity without kicking your butt through a cardio session. Walking IS cardio! Aim to get in more daily movement, steps, or take walks when you can. Shopping, cleaning, hiking, gardening, riding your bike and more also fall into this category!
Low-Intensity Cardio - Another great way to incorporate more cardiovascular activity into your routine would be to simply add in some light, lower-intensity or steady-state fat burning activities such as walking, light jogging, cycling, swimming, and so on! Aim for anywhere between 20-30 minutes of such activities, 2-3x per week. If you are doing this on the same day as a lift, try to get it in separetly or AFTER your lift. This will allow you to more effectively burn body fat, and subsequently preserve your energy stores and muscle tissue for your weight training!
Cardio & Your Metabolism:
In general, if your cardio sessions become more frequent or higher in intensity than your weightlifting sessions (in other words, become the primary exercise stimulus on your body), your metabolism will slow down in order to become more efficient for this type of exercise.
This slowing down of your metabolism (over time) is due to the continued higher stress placed on your body by this type of cardio, to which your body will respond by burning off calories/energy during the exercise to become more efficient, and holding on to what it can afterwards. This may sound like a good thing at first, but over time your body adapts to this and consequently will hold on to more energy (in the form of stored fat) in order to preserve it for the next higher intensity cardio session to come.
When you lift weights, you are sending the signal to your body to build muscle and then feed that muscle, so your metabolism will INCREASE in order to accommodate for the additional intake. Doing cardio alongside of weightlifting doesn't automatically cancel the effects of the strength training, but depending on which form of exercise is the primary stimulus to your body, conflicting signals will be sent to your metabolism. These mixed signals can stall progress for muscle growth and lead to burnout! So if strength training and building muscle is the goal, stick with that first and then supplement with cardio as desired.
Doing cardio as a supplement, or even occasionally throwing in a higher intensity session (I personally love to take a boxing class once in a while!) will not automatically ruin your weightlifting progress or hurt your metabolism. You just want to focus on keeping the metabolism-boosting exerise - weightlifting - as a priority to set you up for success. (PS: If you are in a cut or fatloss phase, supplementing with cardio can be especially beneficial for additional calorie burn & fat loss, but the above rules still apply!).