Circuit Training?

At 75 I commend you for getting back into the gym. At your age it is important to maintain your muscle mass and bone mineral density (BMD). I won't make this to scientific for you. When looking at building muscle or BMD you are looking to stress the body in the correct way. Strength training is the correct way. With your age I wouldn't get involved into a program that is confusing. Circuit training is fine. What you need is a system that progressively overloads your body. Pretty much what that means is that as you get stronger you need to increase intensity. The body adapts to stimulus, so you need to keep adjusting the stimuli.

Traditional methods most likely will be your best bet. When looking at set and rep ranges stick to 2 or 3 sets of an exercise. When looking at a rep range keep this in your mind. 1-6 reps are for strength, 8-12 are for muscle building, 15+ is for endurance.

The main takeaway is that you need to provide the correct intensity (weight) with the correct rep range. Starting out I would build a base doing 2 or 3 sets of 8-12 reps. When you are able to do all of your reps you should increase the weight. When you become a little more conditioned in the coming months start adding some heavier weights with lower reps. Popular methods are 5 sets of 5 reps or 3 sets of 3. This is good for strength, but it is also great for BMD.

Others will have ideas and feedback for you as well. This board has a bunch of knowledgeable posters on it. I think the KISS method would be the best for you (KEEP IT SIMPLE STUPID). That is unless you become a 75 year old gym freak, which does happen, then I would suggest some more intense progressive overload programs.

Hope this helps and good luck on your journey.

No worries. Happy to throw out some advice. One thing I forgot to address was your question about time between sets. There's a couple of different schools to this, and it all depends on what you want your outcomes to be. If you are looking at building muscle (8-12 reps) keep your breaks pretty short. Thirty seconds to a minute. The build up of lactic acid (the burn you get in your muscles) sends a signal to your brain to release hormones, mainly insulin like growth factor (IGF 1), human growth hormone (HGH), and increases testosterone production. If you are looking at building strength (1-6 reps) I would recommend at least 3 minutes between sets. Studies have shown that at about the 3 minute mark your muscle cells have recovered about 80% of their creatine stores. Creatine is an important part of your muscle energy systems.

Hope this helps.