I basically do two types of general fitness progression runs moderate and hard. Lately I have been doing a lot more moderate progressions but over the course of my running career my single favorite workout is the hard progression run. Like the names suggest the difference is in the effort level. To be honest the effort throughout most of the run is the same it is just that at the end of the hard progression run you keep accelerating until it gets very hard and you can't keep increasing the pace anymore.
The length of this workout is entirely at your discretion. I would suggest that for 5k runners you want a run of at least 20 to 30 minutes for the 10k you want 30mins to 1 hour. For the half marathon 45 to 90mins. For the marathon, if you are using these as a general fitness workout you should stick to the 1 hour to 90 minute range but if you do want to make it more specific you can extend them out over 2 hours.
The idea is simply enough start out running a bit faster then regular training pace and steadily increase the pace a little bit at a time until you are running just about as fast and as hard as you can. If you do these workouts enough you will find that your body reacts to getting tired by picking up the pace. You will find yourself coming apart in a tempo run and hitting faster and faster splits, which only quicken your demise but it is neat all the same.
I think most often people pick a normal training run route and pick up the pace each mile or kilometer. This is fine but I suggest a different plan. I would strongly advice you pick a loop, the length of which is not important it can range from 1/10th to 1/3rd the length of the progression you plan on doing. Personally I like to pick a loop that will have me taking about 5 to 8 loops for my whole workout. This loop does not need to be measured. The terrain should reflect your goal race terrain. Next make sure the loop is one that you will not have to stop. IE no intersection crossings- at least not ones with traffic- no train tracks etc… Ideally a trail loop with good footing is available but right now there is ninety something inches of snow on the ground here so if you experience similar situations then you to will have to find a road area.
The reasoning for this is simple. The idea of a progression run is to be constantly increasing the pace. It is hard to know if you are really doing this if the terrain is different. On a 10 mile loop comparing mile 1 to mile 5 is often apples to oranges. However if you do 7 loops of 1.35 miles or whatever each loop is they are each exactly the same so you know if you run each loop faster than the one before it you are correctly increasing the effort.
So you have your loop. You run the first loop at about or just a little quicker than your normal training pace. (I would do a warm up before this but I can't run much under 8 or 9 minute mile pace without a warm up but I have been running over a 100miles a week with some pretty darn good consistency since Bill Clinton was President and that doesn't come without a price). On each loop you increase the pace from the lap before. How much? As long as it is faster than the loop before you have done your job. Don't over complicate. Just don't increase too much or your workout will end up being a lot shorter than you planned.
Each lap you get faster until you reach the final lap where you should steadily accelerate over the whole lap trying to finish at a full kick sprint and around a maximum effort. If you find yourself at a point where you know you can't run the next lap faster then the lap you are currently running then the workout is over. You need to just hammer to the end of the loop you are running. DON'T slow down and try to salvage. The lesson of constant increase of pace is more important. So your planned 90minute progression run ends up only being 45mins. That is ok. It is still a good workout. It still taught your body what it needs and it has steadily increased the pressure on your heart and the amount of acid your body had to process out of the blood. Also you did your fastest running when your legs were at their most tired. Next time you will pace yourself better and do the volume you want for the workout and that too will be the successful learning of an important lesson on understanding and controlling your own body.
A few words of caution and guidance. Don't pick a loop that is too short. If you want to do a 1 hour profession run and you run around a loop that takes about 3 minutes to run you are setting yourself up for failure. It takes some crazy pace sense to only go 1 or 2 seconds a lap faster each lap. So what invariable happens is you drop 10 seconds or more on one lap and after you do that a couple times you are running crazy fast and you are done. Frankly if you are running the first loop at 3 minutes at a nice steady pace it is unlikely you are capable of running that loop all out under 2 minutes. Better to do a loop that is a little longer and give yourself a bit more time and distance to play with. I also pick a couple spots on the loop to use as pace checks, not too many though as you mostly want to be focused on feeling smooth and covering the ground quickly.
Next make sure the footing is good. This is not the time and place for technical trails or tight turns. You want a nice steady effort and fancy footing and breaking hard and re-acclerating are not conducive to that.
When to do this workout? This is a great base phase workout for all distances. It is a great anytime workout for half marathoner and marathoners. This is also a great workout for maintaining aerobic fitness in the specific phases for 5k and 10k runners. Also it can be used as an aerobic focused specific workout for the 10k and cross country. Anything 5k or shorter and something this aerobic is not going to be very specific. You would be better in that case doing a portuguese surge, http://nateruns.blogspot.com/2015/01/tempo-tuesday-portuguese-surge.html
This session will build the physical and mental ability to run relaxed at a quick pace and to make a long hard killing drive to the finish of a hard race. Doing it once is nice but done regularly this workout will reinvent you as a runner.