The instinct, when the weight starts moving in the wrong direction, is to do more cardio. It is what worked in your twenties and it is what every gym induction still points you towards.
In this decade it works against you, and the mechanism is worth understanding rather than taking on faith.
Why cardio underperforms here
The metabolic change of midlife is driven substantially by muscle loss. Less muscle means a lower resting metabolic rate, which means the deficit you need in order to lose weight gets larger every year. Sustained cardio combined with eating less is a highly effective way to lose weight, and a meaningful fraction of what you lose is muscle. So the next round is harder than the last one.
Resistance training does the opposite. It signals the body to retain and build the tissue that is under threat, which protects the metabolic rate you are trying to defend, and it loads bone at the moment bone loss is accelerating fastest.
What good looks like
Two or three sessions a week. Six movement patterns. Forty minutes. Weights heavy enough that the last two repetitions are genuinely difficult, and heavier over time.
That is the whole prescription. It is unglamorous, it fits in a lunch hour, and it is the single best-evidenced thing available to you right now.