There are many ways to stay in shape -- some better than others, some more convenient. Lately I find myself walking to the gym (another gym rat here) which for some my age would be a work-out all by itself. Once at the gym I hit the weights either legs or upper body. Walking to the gym to work legs may seem redundant, but walking and lifting (heavy) are way different. Squats (mentioned above) are part of leg day but it is a full body exercise.
Quote:
...the squat is a compound, full body exercise that trains primarily the muscles of the thighs, hips and buttocks, quadriceps (vastus lateralis, vastus medialis, vastus intermedius and rectus femoris), hamstrings, as well as strengthening the bones, ligaments and insertion of the tendons throughout the lower body. Squats are considered a vital exercise for increasing the strength and size of the legs and buttocks, as well as developing core strength. Isometrically, the lower back, the upper back, the abdominals, the trunk muscles, the costal muscles, and the shoulders and arms are all essential to the exercise and thus are trained when squatting with the proper form.[1] ...
Proper form is key when weight lifting -- primarily because bad form leads to injuries. Keep the weight down until you master the correct form.
(For those who need to train their heart, when I get to the heavier sets, my heart-rate will go above my theoretical max (220-"your age"). Put another way, my chronological age is more than a couple years older than physiological. I'd bet that many in this forum are the same; if you stay in shape some of those theoretical numbers are meaningless.) Back to leg day -- the walk home is in some ways harder even though it's mostly downhill.
CoQ10 anyone? Yeah, daily LOL

On other days I work upper body, lots of different exercises there, if you pull, don't forget to also push -- meaning, if you do curls to work your biceps, don't forget to also work your triceps. I prefer compound exercises though, Lots of different muscles working together to move weight one direction or the other, the way they work in real life. There's more control in the gym though; it's safer than real life if you watch form.

FWIW, YMMV