RusP, I have been there, the first few weeks of a new routine or schedule are always hard, especially if you are new to doing physical work and a full hours (and then some). However, give your body a chance to adapt and get used to your new lifestyle and you should find that you get your energy back for evening workouts. At the end of the day it’s an office job. Plenty of people have far more physically demanding jobs like labouring, landscaping etc. and manage to get a good evening workout in.
Make sure you are eating consistently during the day. I find MRP shakes and sandwiches the way to go in the office, since it’s a bit easier to get these down quickly during the day while not disturbing your co-workers. Also drink a lot of water, that office air conditioning will take it out of you otherwise.