Q: Is it better to do my workout or runs all at once or split it up into two sessions?

A: Ah, to do double days or to stick to solely singles? The answer depends on how much you are doing and also what the goal of that days' workouts is. There are benefits of splitting your mileage up into two runs, or two separate cardio sessions if you are cross training, and also some for sticking to one workout a day. In the case for doubles there is at a certain point in weekly mileage totals that it becomes sheer necessity; obviously if you are doing high mileage you aren't going to be wanting every single run to be in the double digits. By doing one longer and one shorter run you allow your body to recover more and if you do a hard track or interval session in the morning, doing an easy couple of miles later in the day will also help flush out some of that lactic acid and you'll actually feel better come the next day.

Sometimes even if you aren't necessarily doing high mileage but need an easier day for recovery purposes, doing two shorter runs or workouts can be good too. This can be the case if you are in a taper phase or just feeling run down; you could then do two shorter, 4 mile runs, instead of one 8 miler. If you aren't necessarily training for a running event or race, but are more interested in overall fitness doing double workouts could still be of benefit. If you are cramped for time and it's only possible to fit in 30 minutes in the morning and 30 minutes at night then so be it; but another plus is that you may have more energy during each of those shorter bouts than you would have if you had done an hour straight. You could then go harder, burn more calories, and increase your strength and fitness. There have also been studies that show doing two, more moderate to intense workouts, will cause two spikes in your metabolism in the after burn effect. So instead of your metabolism being elevated only once after a single workout, you'd get the reward of a second spike if your aim is weight loss.

Now, the benefits of doing singles are that they are best at increasing cardio fitness and endurance and they are sometime just more convenient. Personally if my daily cardio or runs were around an hour I'd prefer to do singles just because it tends to be easier time wise and the whole getting ready/showering off. If you are training for a longer race or event you may want to do as many longer, single runs just for that extra endurance training; and many higher mileage folks will do their longer run in the morning and still tack on a few miles in the evenings more as just a flush-out/easy run.

So the answer really depends on what your own goals are, where you are in your training, and what works out for you given your schedule. One final note is that on the days of competition or a hard track or workout session in the evening, it's common for people do do a short run (say maybe 2 miles and some drills and stretching) in the morning just to loosen up and get those legs ready to roll later in the day.