As outlined in the excellent book Fit, the basic physical qualities are strength, endurance and mobility. Mostly I'm interested in strength and mobility, since most Westerners spend their lives sitting on their bums, their strength and mobility are overall atrocious, so improving these has a big effect on how a person looks, feels and performs, endurance not so much. Anyway endurance is trendy so plenty of other trainers deal with that, I'm happy to leave them to it.
But this doesn't mean I just whack a barbell on everyone's back the first day they come in. Often I'll begin with circuit training. This surprises many young trainers who've always been strong. "What's the point?"
Remember that each type of training has a primary benefit, but other secondary benefits. Aerobics will build your strength, low-reps high-weights will build your endurance, and so on. For example, if I get a deconditioned woman of 60-70kg doing barbell back squats, she might struggle to squat 30kg. If she has years of aerobics behind her, she might manage 50kg. That 20kg difference was the strength she got from aerobics; she wasn't trying to get stronger, and the improvement of her VO2max would be much greater than the improvement in her strength, but it did nonethless improve her strength.
So circuit-style training will improve both their strength and endurance. Remember that our neat divisions of strength, cardio, muscular endurance, etc aren't really relevant to a deconditioned beginner. They're just shit at everything, they need to do something. If I can do 10 pushups with two 20kg plates on my back, or 60 pushups without them, you can assess my relative muscular strength and muscular endurance. If I can only do 3 knee pushups, the distinction is simply irrelevant. Whether I do pushups or bench press, high or low reps, this doesn't matter - I am going to improve all aspects of fitness.
Commonly I will have people go to the Smith machine and put the bar at knee height. Away from the bar they do goblet squats, then some pushups, then under the bar to do inverted rows, with no break between the three. A short rest and then back to it. The progressions and regressions of each exercise ought to be obvious. This is "circuit" training, but it will improve their strength.
Or we could do the same in the squat rack, they could do some heavy squats, then inverted rows under the racked bar (most women beginners will be unable to do inverted rows from the horizontal with their legs straight out, from the bar at chest height will be easy and they can do 10-20 reps), then some pushups. So this is a bit of strength work in the legs and postural muscles from the squat, then more muscular endurance with rows and pushups.
I'll also have people do kettlebell work. A minute each of swings, rows, squats and presses, then a minute's rest, then go again. This will improve both their strength and endurance.
Women especially tend to enjoy circuit-style training, they like to be kept moving and puffing a bit. That's another reason for aimless sessions. This dripping with sweat puffing puking tends not to happen with low-reps high-weight until the person gets quite strong. Even a weak person is not going to puff from squatting the 20kg bar, either they lift it or they don't, no puffing involved. Get them doing a minute of goblet squats and they'll be puffing, though - and still improving their strength.
Clients and gym members will sometimes question the PT's training methods, and so they should. A trainer should have a reason for every exercise, every set and rep in the session, and be able to explain where they expect things to progress to. Explaining to them the reasons for it all tends to make them more co-operative. I find phrases like "We train movements, not muscles - do you have to sit down and stand up? Then you need to squat -" are quite persuasive.
