Races are set up in plans to give you feedback on your training. Helps to break up the grind of training, and allows you to test your fitness. Some people use paces for their workouts, and you 5k time can help you to figure out those paces. A 5K time can also help to get an idea of what you can expect to run a marathon at (if you put in the training). A 5K time represents your speed potential, and when you add the endurance component to your speed potential you can accurately predict your marathon time/pace. So when you do run that marathon you pace it properly, based on your volume and speed potential on race day.

4x1mile repeats ALL out, race efforts, with 3 minutes rest in between the miles, will give you an idea of your 5K pace. The average mile time for the 4 repeats will be right around your 5K pace. So in the future, if you can't find a race, do the repeats.

A 5k test can be a very useful tool to predict your marathon time. Having an accurate assessment of your marathon potential can ensure that you pace the first few miles appropriately, which as we know makes a huge difference in how the race feels after mile 20.

Also, the 5k test can be used to calculate speeds for track workouts and tempo workouts. Instead of just doing them by feel, you can do them at particular paces designed to make you faster in the long run. I highly recommend the book "Run Less, Run Faster" which has a ton of workouts geared to the marathon training plan, and they all base off of your 5k pace. When the workouts get too easy, you do another 5k and if it's faster, you've earned the right to work even harder!

well any speed work is beneficial even for a marathon runner...boosting your anerobic capacity is still part of distance training

As others have noted here, the true benefit of "B" races within your schedule are to give you an idea of pacing and endurance. At this point in your schedule (week 5 out of 16) you are looking to figure out what type of pace you can hold comfortably. Remember that right now you do not have the endurance built up for a full marathon (...yet), so you need something much more relavent to the miles you are putting in at this time, hence the 5k.

In addition to the pace aspect, there is a confidence aspect that shouldn't be over looked here. One very important part of training for a marathon, or any long distance event, is to stay motivated! If you are sticking with your training, the results will show in your races, and that can be a huge boost of confidence. Never underestimate the power of achievement!

Keep up the good work, and if you need some help please feel free to check out my blog for some inspiration at TriathleteTalk! Good luck :)

The main difference (I find) between a "mock" 5K race and a "real" 5K race, is that when you are running by yourself the times are slower due to the effects of drafting that you have in a real 5K race. I personally find it really difficult to use these times for pacing due to this effect. I prefer using PRE, to judge effort. No it is not an exact science, and I do have trouble making goals such as I am going to finish "X" race in "Y" time. It ends up being more like this: I'd like to finish this race in "Y" time or less, according to the calculator it is possible etc. The calculator I am referring to is the McMillan calculator.

http://www.mcmillanrunning.com/index.php/site/calculator

Skipping the 5K race on the plan is fine, because you wouldn't run a marathon at that pace anyways. And if you have done many long runs you will get a sense of what your pacing will be. Again though, using PRE levels any effects due to drafting, that is why I like it.

If all you're doing is long slow distance workouts, definitely buy the Run Less, Run Faster book. It is an invaluable resource, you won't be sorry!

A race, whether 5K, 10K, 15K or what-have-you is always going to be a bit faster than doing it yourself. The motivation in a race is different, the crowd motivates you, and the adrenaline boosts you. You really can't 'mock up' a race very well IMHO. Usually a good plan has a race or two in there somewhere. You can always move races around a week or two if you can find one that doesn't perfectly line up. You can also race and then add any extra miles you need if your day calls for X miles and the race is shorter.

I have found that trying to run my long runs aerobically paced has actually been detrimental to my training and has actually slowed me down. I've found aerobic paced training is best left for base training before starting a defined plan unless it specifically includes base.

[quote=Ironmom]If all you're doing is long slow distance workouts, definitely buy the Run Less, Run Faster book. It is an invaluable resource, you won't be sorry![/quote]
especially for triathletes who generally are not going to run 5-6 times per week. I really try to follow the Run Less, run Faster approach and the only change is throwing in a brick just to maintain my familiarity with appropirate pacing off the bike.

All the things mentioned are great, but lets not forget the physiological benefits of a 5k race. What effort does the plan say to run your 5k? If it says to do it at 15k pace, that's tempo run pace. If it says all out, that has the same benefit.

Just to drop a few respectable names of coaches, Jack Daniels is the one that said running faster than tempo pace will still raise your LT. His words - "A tempo run is a steady, controlled run that should last about 20 minutes at threshold pace. A steady intensity of effort is important. Going too fast on a tempo run is no better than going too slow, and neither is as beneficial as running the proper pace. You could perform a longer-than-20-minute tempo run, but 20 minutes has been shown to produce positive results, and it will leave you relatively fresh for the next day's training."

Pete Pfitzinger said "Athletes racing from 15K on up to the marathon receive the most benefit from tempo runs because the physiological adaptations are most specific to the demands of those races," he notes. "An improvement in lactate threshold is only a small benefit for a 5K race because that race is run well above lactate-threshold pace. Performance in races of 15K to the marathon, however, is determined primarily by the runner’s lactate-threshold pace."