Well, before 2002, I always believed that most runners would be well served using their marathon training to train for a 10k because that is still far less than optimal 10k training. However, I felt that, once you are nearing that maximal training load, you are better off doing a little less volume and a little more intensity.
Then, I had an interesting experience in 2002. I was definitely marathon training, doing more volume by far than I had ever done before. I averaged 120 miles per week for about 4 months and, while I backed off a bit on the amount of intensity I was doing, when it was time to run fast, I really made it count. Without even tapering, I ran an 8k PR (this is a distance I’m no stranger to) with a good but not great race and I seriously felt that I was in PR shape for all distances from 5k up.
Not comparing myself to someone who is out of my league but you can notice the same trend in Paula Radcliffe’s running. When she started marathon training, her times on the track improved after being relatively consistent for some time before.
If you look at what Lydiard has runners do, much of the training is the same whether your goal race is 800 meters or the marathon. Considering the fact that his first Olympic gold medalists were in the 800 meters and the 5k, this should tell us something about what marathon training can do for you.
In case you can’t yet figure out what I’m trying to say, I believe well planned marathon training can bring a person to PR shape in everything from 5k up and possibly for even shorter distances.