I constantly see guys killing themselves at the gym. Too many sets, too much time on the treadmill, or straining themselves to lift way too much weight. That last one will hurt your back and make you miserable.
I use a simple theory- keep the workouts short but steady. One machine to another with little time in between. Do HiiT training to warm up, if you need it, and maybe some moderate stretching for keeping the back limber.
Train 4-5 times a week so you have enough rest to recover fully.
Thats it - If you want an example here:
Warm up- Cycle 4 to 5 minutes, 30 seconds moderate, 20 seconds at max pace.
Ab training- alternate Leg raises with machine crunches. Use as much weight in the crunch as you can for 5-8 reps. After a warm up set with moderate weight for 8 reps.
Day one - Legs, Back , Bicep. Use machines or free weights, and after a moderate set to warm up, use max weight for 6-8 reps or failure. Whatever your choice of exercises are. Change these frequently to keep it interesting and different.
Day 2- Chest ,shoulders, triceps.. Same idea as above. AB work, cycle, then you choice of routine for these muscles.
I take a sauna afterwards for 15-20 minutes, Shower and done.
The whole thing takes 45 minutes .... IT works!
Combined with proper nutrition this can make a big change in your life without killing you, or taking hours and hours to get through.
Remember to max out on the weight after the initial warm up on each machine or movement.
You should only be able to get 6 or so reps before you fail.