New to training - fighting work fatigue

You'll have to devise strategies that work for your life style.  I"m able to run at work during lunch hours which breaks up and reduces the workday's stress.  I also run late at night.  Most of my bike workouts are done on days that I decide to commute by bike, so again, it's almost like part of my work day.  Long rides I do by getting an early start, so that I'm back as family is getting their day started, on an available weekends. Swimming I'm able to do when I take kids to the pool. If I feel fatigued or physically depleted, then it's time for an extra day or two off.  Rest is essential to help build endurance, so I'm long past fretting about not working out.  I just tell myself that my body is getting stronger as I rest.  When I'm rested and fit, I don't feel fatigued even after a long day at work.  

This may sound like a "duh!" statement, but I've found the more I get out and exercise, the more I want to. I feel better. And to second Ken's response, don't overdo it. You don't have to work out hard every day. On the days I know I need a break, I walk.
Depending on what time you go to work, if you're a morning person, etc., you could plan a morning workout. The key to sticking with this is to have fun - and don't kick yourself if you miss a workout
Also, just thought of this, try to eat a snack in the afternoon. It'll help your energy levels. We all need fuel!
Welcome to BT!

Again, this may sound really obvious, but: make sure you're getting enough sleep on a consistent basis. If I'm sleeping enough and eating enough, my workouts don't wear me out. I also try to add an hour of sleep the night before a particularly long workout, like a long run.

Welcome to BT!