I am in a relatively similar position.
Granted, I love my current job but, like yourself, I just find myself too tired to do any proper exercise. To make matters worse, whilst I don't go crazy on breakfast and lunch, when I come home, I tend eat a lot for dinner. Coming from an Asian (oriental) background, this involves rice - and I consume a lot of it!
Prior to being employed at this job (which I've only had for just over a year), I was pretty much unemployed for around 8-9 months with a brief stint at part-time employment towards the end, I have been very good at controlling my weight; I have had the energy to go for regular runs/long walks/sport and have also been able to moderate my calorific intake.
I'm aware that the drive to lose weight involves a lot of mental strength, but I am struggling to do so at the minute.
How do you full-time workers shape your day to incorporate a good diet and regular exercise?