Ever since I got an office job I sort of had the same problem as I didn't even walk around or anything and I just didn't find strength to go to the gym. But I've been trying to follow Kayla Itsines and her training program which can be very easily be done in home and just for like 30 minutes per day. So I suggest you to check or someone similar out who provides fitness programs that you can do without going to the gym.

Still, it requires to find motivation to get up and move, but once you've forced yourself through the first or first couple of weeks, it will already come naturally that you want to keep on doing that