Firstly, I don't have personal experience with it, but the book Convict Conditioning is popular. I took a quick look at the first review on Amazon and it sounds solid. Body weight exercises sound like a joke, but the unilateral shit like one arm push ups, chin ups etc are nasty movements. It would also let you avoid gyms completely. Secondly, you don't have to worry about getting huge. It doesn't matter how hard you work out or what you do, if you don't eat significant caloric excess than you will not gain significant size. Body mass, whether it be fat or muscle, equals calories, period. Thirdly, barbell compound lifts are very big on the internet because they are effective, efficient whole body work outs that improve strength head to toe. Every able bodied aging male should absolutely be performing them to maintain and improve muscle tone. Males lose a scary amount of muscle mass each decade beginning at 30 and combating that loss is a BIG BIG deal in improving ones quality of life. I would recommend reading Starting Strength if you develop an interest beyond body weight exercises. Or at least read the book's foreward. Lastly, lol @ the runners you mentioned. That is exactly what I think about when considering the aging cardio addict. Terrible muscle tone, busted legs, high body fat on a slender frame. Low back pain standing? Bad posture? I work with a literal fatty who somehow "bikes" aggressively and his physique makes me nauseous. I'm no expert when it comes to diets and I don't understand how to "get shredded" long term while gaining strength, but I do believe that it is EXTREMELY difficult to run into middle age and expect to be lean, especially when the inexorable loss of muscle requires the body layer on visceral fat to fill in the spaces.