Sixty-four year old here. My main goals are cardiovascular fitness, protection of joints, muscle & bone mass maintenance and flexibility in that order. To achieve this, I've been working out since my forties when I gave up clinical nursing which involved 8-12 hr days on my feet, moving patients and running to perform CPR, etc.
Five days a week I run 30 minutes on an elliptical which I regard as better than a treadmill or street-running as your muscles are working while your ankle, knee and hip joints are supported. As I age I will change to a stationary bike to offer more support and then, as decrepitude inevitably continues, start swimming and water aerobics.
Another 30 minutes is spent in stretching & moderate weight-lifting - six different muscle group sets of 3x15 exercises where the weight is just on the outer edge of comfortable. I'm not going to be all "muscly" but, I feel solid to the touch and have decent definition. Use machines rather than free weights as they guide correct repetitions. It also makes a spotter unnecessary.
We are built to use our bodies but I fail to see the necessity of gaining massive bulk. If that floats your boat, all well and good but, I wonder how most 30-yr olds with huge muscle mass will fare as they age. i have had no major injuries beyond persistent, mild sore muscles which feel better after 800mg of Ibuprophen and the weekend away from working out.
I've built my fitness slowly and persistently, not competing with anyone but myself. Regular, gentle exercise with a very gradual increase in speed, weight and resistance have been the keys. Also, recognizing that doing less weight than you're maximally capable of lifting will maintain joint & muscle integrity longer. Boredom is probably your worst enemy after you have willed yourself to exercise habituation so, find some form of distraction. I avoid it by listening to EDM on headphones. The beat keeps a fast rhythm and the music is distracting enough to make the time go quickly.