If you feel like you're lacking energy it could be that you're not eating enough calories during the day.

I took up resistance training a few months ago and it required I increase my daily calorie consumption by about 500kcal. I only realized how little I was eating after using an phone app to meticulously record everything I put in my mouth on a daily basis for a few weeks.

Since I started it really forced me to learn a lot about human physiology (because you can injure yourself if you do it with bad form), nutrition (because I want lean mass gain and optimal recovery), I sleep more (recovery) and stopped smoking. There's an amazing euphoric feeling after a heavy weightlifting session when you've got a good pump going. It can be quite a "technical" sport once you start factoring in all these things. The actual process of lifting a heavy object is the easy/least time consuming part.

Just yesterday I completely changed my sitting posture after I was having trouble with my infraspinatus muscle (bad posture at the desk). Now I'm finally sitting up straight at work because it negatively affects my weight lifting if I don't. Man there are so many benefits...