I’ve just joined a gym (first time ever!) and want to tone my bum, upper thighs, and stomach, as well as strengthen my heart and improve my general fitness. I’ll probably only have time to do 2 short sessions a week in the gym, plus a bit of swimming. I’m currently early 40s, slightly underweight, and of average fitness. I want to avoid losing any weight. What are the best machines/techniques etc to use?