At age 73, I guess I qualify as past 45.
I've been working out at Defined Fitness three times per week, for about 2.5 hours each time. That does not include aerobic exercise since I normally ride my bicycle 5.2 miles to the gym for a total of 10.4 miles each time. The only times I get aerobic exercise in the gym are if I drive my car or ride a motorcycle to the gym in which case I use a stationary bicycle for a few minutes just to warm up. Also, about once per week, I ride my bicycle for a longer distance, usually 30 miles. Doing aerobic exercise on a stationary machine can be effective, but I can't do it except briefly; it's too boring. For the same reason, I can't swim laps.
In warm weather, by the time I get to the gym, I'm all sweated up so I take a shower and change before working out.
At the gym, I use a combination of machines and free weights. When I changed to Defined Fitness from the gym for geriatric citizens (the price was right, $13 per year + a $1 donation each time), I used a trainer for six sessions to get me started. I may again use a trainer to ensure that I'm doing everything properly, but I see no reason to use a trainer on a continuing bases, especially considering my budget.
I'm hoping that all this exercise will delay my going to a nursing home.