Do I have to get my daily exercise all at once?

Not at all! If you can’t devote a longer single time block to exercise, splitting it up over the day or over the week will still yield great results and benefit your health just the same. When it comes to incorporating cardio exercise into your day, it’s been demonstrated that accumulating a total of 30 minutes per day (for example, three 10 minute bouts in a day) is just as beneficial to your health as 30 continuous minutes of the same exercise.
When it comes to strength training and cardio, if it’s difficult to perform them one after the other, split it across the day or even perform them on different days. You’ll still reap the benefits of both. Exercise is about increasing activity in a systematic way, it doesn’t mean killing yourself in the gym to get it all in at once. Recovery time is just as essential as the workouts themselves so be good to yourself and pace your activity and spread out the workload.

Psychologically it's ok to break up your workout into two or three shorter workouts versus trying to get it all in at once. Shorter workouts actually provide incentive for people to workout more because it's a shorter amount of time that fits better into busy schedules, the discomfort is for a shorter amount of time and a shorter time frame really helps people to feel like they can get through it.

Mounting scientific evidence suggests that shorter, more frequent exercise sessions can be just as beneficial for your health as longer sessions. In fact, one study funded by the National Institutes of Health found that short-but-frequent workouts (10-minute exercise sessions, four times a day) produced the same health benefits as a daily 40-minute session. So, if time has always been a barrier to getting in physical activity and exercise because you thought you needed an hour at the gym to make your workout count, you can now take comfort in knowing that it is possible to improve your health and fitness and manage your weight if you choose shorter exercise sessions but perform them more often. Put this into practice by taking a 10-minute walk in the morning, both before and after lunch, and in the evening for a total of 40 minutes of physical activity every day.

Not at all, it may be hard to bread away and fit in a 30-60 minute workout. You may only have 15 minutes in the morning to do a fitness DVD, 15 minutes at lunch to take a brisk walk, and maybe go for a short jog or walk your dog in the evening.  This all adds up and counts!  Anything is certainly better than nothing.