There are so many excuses for not working out and I have heard plenty in my day. If the reason you stay melded to the couch is that fitting exercise into your life is not as easy as it used to be back in high school then you are not alone.
According to Steven Bray, a researcher at Canada's McMaster University, the inability to cope with change is one of the biggest reasons our activity rates decline as we age. Back in high school many people play organized sports, bike to school, and take required gym classes. During college and the years following, physical activities are harder to come by and harder to fit into schedules.
This is why it feels like you have to double your efforts to stay active, compared to when you were a high-school student. Don't give up hope though. Make a fitness plan, get a workout buddy, join a team, join a running club, take the stairs. I found that once I carved a place for fitness in my life, I can't really maintain my bearings without it.