Up until recently, Weight lifting was considered a fine exercise for strength/Flexibility and appearance. Recent studies however have now confirmed what some suspected for all along that weight lifting also improves cardio heath independent of traditional cardio work. And you don't have to use heavy weights or spend hours in the gym every 2'nd day. But you do have to put forth some effort. Using a 20 lb dumbbell when you can lift a 30 lb and still complete the exercise won't cut it either.
Being retired and not terribly active otherwise, I am trying to get into the habit of doing 3 x 30 minutes sessions of maintenance level weight training using dumbbells to 1/2 my weight per pair followed by 35 minutes of cardio on the treadmill at 55 - 70% of my heart rate reserve, and 45 minutes for the 4'th day on the treadmill.
While I really enjoy biking, Whether permitting, Where I ride there is no way I can sustain the 55 - 70% of my heart rate reserve for 35 - 45 minutes like on the treadmill. And even if I could it would take all the fun out of my bike ride. So for now, Any benefits from biking are just an added bonus.