If you want to gain maximum benefit from your exercise program, you should try to do both endurance (heart-lung) activities and resistance (muscle strength) training. The usual way to do this is to:
• do your endurance training with an activity that uses primarily your legs, such as running, walking or cycling
• use weight-training machines aimed at strengthening primarily your upper body and core (belly and back).
If your main sport does not focus on your legs, you will want to do some resistance work for leg strength as well.
How to Combine Strength and Endurance Training
Researchers in Australia showed that adding a weight lifting program to cycling or running will help to improve performance only if you know that when your muscles are sore, you have to take the day off or go slow and easy (Sports Medicine, July 2017;1–14).
• After running or cycling very intensely, you need to go slow and easy for more than 24 hours.
• After lifting weights intensely, you should go easy on those muscle groups for several days
If you want to train the same muscle groups for both endurance and strength, you need to use long recovery periods of less intense exercise after each intense workout, or you will be at high risk for injuries and an over-training syndrome of chronic fatigue and muscle damage. You should not exercise at a fast pace or lift heavy weights when your muscles are still sore from a previous workout.  High-intensity interval cycling done after heavy-resistance exercise decreased strength gains because of the soreness it caused (Scand J Med Sci Sports, Sept 23, 2016).
You can set up a schedule for lifting weights every second or third day and alternate running or cycling intensely on one day and easy on the next, but always listen to your body and back off if your muscles hurt. One study showed that in just six days of training the same muscles intensely, your muscles will be so sore that you won’t be able to exercise intensely and you will have a significant decrease in strength (Research Quarterly for Exercise and Sport, Aug 2015;86(4):387-396).
If you are an athlete who competes in total body endurance sports such as rowing, cross country skiing or swimming,  you should do your resistance training on the same day that you take an intense workout.  Then you may need two or three days for recovery.  Cross country skiers and rowers have the highest rates of fitness and modern runners and cyclists are now  following their training methods, with more than 87 percent of their workouts being recovery and about 10 percent being very intense.
My Program (84-Year-Old Man)
• Strength Training: I try to go to the gym three times a week and use the upper-body weight machines. I do only one set with a comfortable weight until my muscles start to fatigue. On most of the machines I do 20 to 100 repetitions. This type of training does not give me large muscles, but it does help me to maintain strength and avoid injuries.
• Endurance: On Tuesdays, Thursdays, and Saturdays, I cycle 25 to 30 miles on a recumbent tandem with Diana in our tandem bicycle group. We do not go flat out but we do pick up the pace at the end of each ride. On the other four days, I attempt to do intervals on my upright single bike. My legs usually feel stiff in the morning. If they do not recover after a 5 to 10-minute warm-up, I either ride 10 to 13 miles at around 11mph or take the day off completely. I usually have to ride slowly two or three times a week because of muscle soreness.
My interval workouts are done fast enough to make me short of breath, but they are not done at my maximum effort. I do 50 half-pedal strokes at about 80 percent of my maximal heart rate.  Each interval takes 24 seconds and I go about 400 yards.  I do not start my next interval until I feel that I have recovered completely from my previous interval. I stop my workout when my legs do not recover a few seconds after I finish an interval. I usually do between 21 to 24 repeat intervals. See Short Intervals are Best
Caution: Intense exercise can cause a heart attack in a person who has blocked arteries or heart damage. Check with your doctor before you start a new exercise program or increase the intensity of your existing program.
Checked 6/5/21