At this point, having just joined a fitness club, it is important you remember one thing: you are only as strong as the job you do. Fitness training in any guise will work your body in ways you never thought of.
For the first few weeks, your body will begin to adapt to the physical, mental and mechanical pressure you have placed on it. Expect to feel very stiff — walking like a duck — even when using the lightest weight. Weight is not important. The correctness of form is everything.
Do Not Train Every Day
Three times a week is ideal. Your body changes when it rests — not during training. It must have time to recover. Injury does occur, even to experienced individuals who push beyond this. Do not be in a hurry to do more than your body is ready for.
Your body changes when you relax — not during training.
What Training Does for You
In the 26 years I have been in this industry, many things have changed — not least attitudes. You will no longer be thought silly for going to the gym, no matter your age or size.
Medically, training helps you in more ways than most people expect. For women in particular, resistance training — using weights or cable machines — strengthens the muscles which in turn exert pressure on the bones, making them denser. This helps prevent brittle bones. Beyond that: flexibility improves, posture improves, and everyday movements — standing, sitting, walking — become easier. It enhances your body's total wellness.
Fitness also aids recovery from medical conditions, both before and after surgery.
Stay Away from Machines — For Now
I would suggest staying away from fixed machines on the gym floor for at least the first few weeks. They are designed to move your body strictly along a set path — which sounds sensible — but your muscles have not yet developed enough strength to pull or push evenly. What happens instead is that you compensate: working from your stronger side only. This is bad technique, and it can be dangerous.
For at least the first three weeks, use free weights — dumbbells and cable machines. These demand more of both sides equally, and will develop your strength in a balanced way.
Work the Whole Body
Initially, work the whole body from top to toe in every session. This approach helps your muscles begin to understand what is required of them, building endurance, strength and flexibility across the whole frame. It will also move your weight faster and increase overall stamina far more efficiently than isolating single muscle groups too early.
Only do what you can do on any exercise. That last repetition — the one you are struggling through — is most likely the one that will hurt you.
There is no medal for grinding through pain you did not earn yet. Leave a little in reserve. Come back stronger next time.