Hi James,
If you walk all day for work, it might be a start to get a Pedometer to track how much exercise you get just from your daily routine.
Once you have a baseline for what your exercise is, get a baseline for your Food intake as well (i use TheDailyPlate.com, others have used FitDay.com, etc).
Some advice that has been widely touted as THE way to start your new life is to increase the number of meals you eat in a day to 5-7 meals a day (this kickstarts your metabolism), but be sure to keep your intake at each meal appropriate and within the range for your total daily intake.
In order to accomplish this myself, I went out and got a bunch of reusable plastic containers that are fairly small (5inches square by 2 inches tall) and make a bunch of soup or chili at the beginning of the week then put a portion of soup into each container and stuff them in the fridge/freezer and eat them throughout the week.. eventually i got enough variety in the freezer so i could eat a different thing at each meal during the day. Heat them up at work, Get home and toss the containers in the dishwasher - easy cleanup!
In terms of your energy level at the end of the day; you may already be doing enough exercise during the day to start down the path to loosing weight (which is why getting a baseline is so important). Combined with the right diet and your existing exercise level, you WILL loose FAT. As you loose, you'll have more and more energy at the end of the day (I promise!). Also, good shoes during the day help more than you think.
I find that those days when I dont have the energy to do much when i get home at the end of the day, listening to some high-energy music (!) or looking at myself with my shirt off in the mirror (observing my fat-ass usually motivates me to DO something about it) will usually get me in the mood for exercising.
Tip: if you do start exercising beyond your daily routine, dont OVER do it so you're extremely sore the next day (We're not teenager's anymore!), start with something that seems too easy (for the first 2-3 days) then move up VERY slowly to something that leaves you feeling your muscles but not very sore: that should be your off-day exercise level. I try to do an On/Off day exercise routine (so i dont get bored, and dont over-exert myself), On day = twice the reps of the Off day (or +5/10 lbs per exercise, depending on whether you think you need strength or aerobic training).
Another factor is what do you LIKE to do for exercise (weekend fun)? Bike riding? Hiking? Stairs? Rowing? Lifting? Start with what you like, and if you dont know what you like yet thats ok too - there are plenty of things to do to exercise. Sticking with it is just a matter of finding what doesnt make the thought of doing those exercises burdensome.
Sounds like you're motivated and are off to a great start!