The long run is one of the standards of almost any running program and like intervals there are an infinite number of ways you can adapt it for your current needs. I am in no way trying to cover all the possible long runs here but instead I want to put some basic info on how you can use them, how long they should be, how often you should use them, what you can expect to improve by doing them.
The long run is a very subjective thing. For me my normal recovery run is 20 kilometers, about twelve and a half miles. For my wife a 12 mile run is certainly a very long run and she treats it as such in her training cycle. It is important to remember that all training is relative. Heck even miles don't mean the same thing to different people. Frank Shorter famously did a weekly long run of 20 miles or two hours, whichever came first which was almost always the 20 miles. Most runners I have coached couldn't imagine running 20 miles in 2 hours. You just can't convince me that running 20 miles in 2 hours is the same as running 20 miles in 4 or 5 hours. Frank Shorter did not wake up Monday morning nearly as beat up after his 1:58 20 miler the day before as John or Jane Jogger is going to after a 4 hour 20 mile run. Just the extra time on the feet has a huge impact on the difficulty of the task and the amount of time you can expect it to take to recover from it.
So what is a long run? For me I consider the long run to be the longest run you do in a micro cycle, for most people a week. This means I put no preconceived notion that a long run must be at least 5 or 10 or 20 miles. A long run is a run that is longer than your normal training distance. So for me today that is say more than 18 miles or so. For me at age 16 that was 7 miles or more. It all depends on where you are now.
So how long should your long run be? That depends most directly on two factors what event you are training for and what your current weekly mileage is. If your goal event is not a marathon than you should keep your normal long run to around 20% of your weekly mileage. These means unless you are running around 100 miles a week you shouldn't be knocking out the famous weekly 20 mile long run. Remember the same folks who made that famous ran 13 times a week and well over 100 miles a week. It is folly not to adapt 1 part of a training program but adapt all the others. If you run half as often, 6 to 7 times a week and half as much, 50 to 60 miles a week it makes sense your long run would be half as long.
For the high school student and weekend road warrior focused on 5k and 10k races who is running 40 miles a week you are looking at a long run of only about 8 miles. Does this mean that you can never run further than 8 miles? No it just means you are probably putting too much emphasis on your long run as part of the training cycle if you are knocking off a 12 to 15 mile run every weekend and that it is likely taking away from your training. One of my least favorite quotes in running is "long slow miles makes long slow runners" but like all oversimplifications it has a grain of truth to it. If you put too much effort into running long and slow when you goal is to run a quick 5k you are not going to be happy with the results.
Now if you are focused on the marathon you can push the long run out to about 25% of your weekly miles. So for the adult looking to run a marathon on about 40 miles a week you can get out and do 10 miles for your regular long run each week. Again this does not mean you should never go longer but outside of the marathon specific phase, which I'll cover in a marathon specific long run blog, you shouldn't be regularly exceeding that distance by very much.
How often? I mean if you plan your training weekly you inherently are going to have a long run every week. One run has to be your longest. That said I don't think you need to make a place to do a run of 20 to 25% your total volume every week of the year if you are getting ready for 5k to 10k races. Too often it is seen as a fall back and a standard of training. "I need my weekly long run" To which I reply why? If you reply I need it because so and so does it or that is what runners do. Well than no. If you reply I need it because I'm in my base phase and/or I'm trying to greatly improve my aerobic endurance right now than yes right you are get off your butt and grind that sucker out!
Remember you should always have a reason for your training. I spent years unable to run more than 15 or so miles at a time, less than 15% of my weekly mileage, because of a nerve issue in one of my legs and I ran most of my pr's from 800m to 10k in this period without any long runs. Could I have run faster with long runs? Yes. Would it have made me massively faster over 5k or 10k? No. There are in fact other workouts I would have much rather been able to do than a weekly 22 mile at moderate effort.
Now that said at that point in my running career I had years and years of high mileage and long runs under my belt and I do think that those workouts done years before my best racing had an impact getting me to that level. If I had been unable to long runs from the start of my running career do I think I would have run as fast as I have at this point for 800m to 10k? Absolutely not and that ignores the events that it would help the most the longer races. Which brings us to…
Why do long runs? When we talk about long run workouts, i.e. long progressions, long tempos, specific marathon workouts, semi specific long marathon tempos etc… they are workouts with very specific fitness gains but when we talk about the just go out and run for 2 to 3 hours at a moderate effort what you are doing is building basic endurance. This can be a finite area to improve in but it is an area that almost all runners could use to improve. Long runs can be replaced with generally high mileage but I honestly feel when you are in the stage of your running where you are trying to build up your basic endurance, I'm talking to you high school and college runners as well as post grads who haven't run say 25,000 miles in your lifetime yet, a good long run can in one day do as much to improve your general endurance as a full week of mileage without a long run.
Now if you are past that level and have a well built endurance base the moderate long runs need only be done here or there when you aren't doing long workouts to maintain aerobic fitness and muscular endurance but it really shouldn't be a focus. If however you are still a developing runner who hasn't banged out years of big miles regular long runs can be a key tool to quickening your aerobic development towards its full potential. Since real success in running is most directly tied to your aerobic ability that makes it a pretty powerful tool.