I've trained hundreds of clients who didn't inherently love working out like me, and who dealt with loss of motivation, injuries, and life getting in the way. So here are my best tips for training longevity:
A lot of people lose motivation once they stop hitting the gym. The great bodybuilder Milos Sarcev was once asked why he trained 7 days a week (something I don't recommend by the way) and he answered, "If I take a day off, I won't get back to the gym."
I'm not saying that you should train every day. But a lot of people, when they begin to have a drop in motivation, take a week off. And from that point on motivation continues to drop even more. So if you start to lose motivation, keep training. You can change the type of training you do, go easy, use less intensity, less volume, whatever, but keep going to the gym.
That's the big one! When you become a real adult with a full-time job, family, mortgage, and a lot more stress, you find that you have much less time to devote to training.
But you know what? Hitting the gym hard for 20 minutes is a million times better than not doing anything. And in 20 minutes there's plenty of productive things you can do to improve yourself. Heck, you "only" have 10 minutes? You can still reap some benefits.
Thinking you have to train for 60-90 minutes is one of the main reasons people stop training. When my kid was born I trained only 30 minutes four days a week for the first 6 weeks. I still made some progress and when things settled I was able to ramp up my training a bit. But more importantly, my motivation stayed high.
Okay, that sounds cliche but it's true, and not many do it. If you make training one of your priorities, you'll find a way to get it done. I've trained pro athletes, Olympians, pro bodybuilders, and CrossFit competitors. But my favorite client was an average 45 year-old Joe with six kids.
He was a business owner working 60 hours a week and traveling across the continent almost weekly. He always found a way to get his 3-5 weekly workouts in. When we started working together he benched 125 pounds. After 6 months he was pressing 315.
This makes things so much easier for people with a busy schedule. For most people the problem is not only the time spent training, but the 20-30 minute commute to the gym. It adds up.
If you have a home gym it'll be easy to fit a quick daily workout in. And you don't need much to start with. If you have a power rack, barbell/weights and adjustable bench you can get solid results.
Reading about training will keep you wanting to train. It'll give you ideas to try out and will keep the fire lit. Visit T Nation daily. Visit the forums to talk shop. Buy some training books.