Once again, Penelope Trunk is right: excercise is not optional. But she says something that exposes a common blind spot, I think:
Regular exercise requires a careful mental shift. First you clearly prioritize what's important to you, and why. Then you pick a specific time and specific place, and then you convince yourself that going is not negotiable.
I exercise regularly enough that I'm stronger than I've ever been on a mountain bike. I have two bikes, two pairs of running shoes, and there's a shower at work where I can clean up after a lunchtime ride or run. Fitting a workout into my schedule is easy.
But trying to schedule an appointment to check out a local CrossFit gym was a disaster. I rescheduled three times, and finally gave up and told the owner I'd stop by if and when I had a chance.
Why? Because trying to "pick a specific time and specific place" is really, really hard for me. And, I think, for a lot of people. Stuff happens. Kids get sick. Dogs get sick. You realize have to get a Mother's Day gift. And so you simply don't have 45 minutes to exercise.
I contend that this is totally okay.
I almost never schedule a workout for a specific place and time. Instead, I weigh the weather, my to-do list, my energy level, and how long it's been since my last workout, and I pack for the workout I want to do that day. If the stars align, I get to go ride my mountain bike after work. Or I go for a run or lift weights on my lunch hour. I do what I have the opportunity to do.
If I want to ride my bike on my lunch hour, but it might rain (biking in the rain is dangerous and miserable), I also pack clothes to go running in the rain (running in the rain is serene and refreshing). I pack a gym back nearly every day.
The trickiest scenario is when I realize I won't get to exercise tomorrow. By the time I know this, it's usually too late to late to pack a gym bag for today. Having the gym bag always packed means I have some options in this case. I can do a workout at lunch or after work. I just worked out yesterday, and I'm tired? No problem. I go easy.
The key here is that I'm ready for whatever will fit into my day, or my week. I don't set myself up to fail by trying to pretend that my life is more controllable than it really is.