For example, someone who runs the mile in 4:09 might not be able to run the equivalent performance in the marathon(McMillan gives 2:20). Vice Versa: the 2:20 marathoner may have a tough time running an all out 4:20 mile. But, both of these performances might actually be equal. You could debate which is harder to do. The fact is, anything done well is hard to do. If someone is a great piano player, they might not be a great guitar player. And a great guitar player might not be a great piano player. But both are great musicians.
But there's more to this than simply looking at what's the equivalent performance. It's also about recognizing where your strengths and weaknesses are as a runner. I am using myself as an example below, where I put down my PR's and put 3 sources that I like to look at(McMillan, Daniels, and Runners World). What I found was interesting. McMillan and Daniels both "agree" that my current half marathon time is my best performance. Runners World, interestingly enough, rates my 1 mile time as my best performance(barely though....the half marathon is a very close 2nd). You can see all the data below and conversion of times.
10 Mile: 50:32(split)
10 Mile: 50:06
10 Mile: 49:57
Looking at this data, in terms of equal performance, I feel that Daniels and McMillan are more accurate than Runners World. But, what also may be true is that Runners World recognizes more how a distance runner such as myself doesn't need a blazing mile time to be really good in the longer distances. What I can conclude with this analysis is that in terms of performance, it is more difficult for me to perform equally at the shorter distances vs the longer distances. I also can conclude that my prime events are above 10K.
There is another part that is interesting for my example. While I get better as the distances get longer, it seems to come to a crashing halt when I hit the marathon distance. I have struggled with performing well in the marathon so that seems to be where I hit my limit of performance. At least it appears so. I haven't finished a marathon since 2012, however, and one could argue that I haven't really hit my potential in the distance yet. It will be interesting to see where I will finish out as my peak running years in this distance. It remains as the unknown for now.
To a certain extent, I think doing both weaker events and prime events will help a runner maximize potential. I can say that getting my mile time down to 4:21(May of 2015) has helped me become more efficient, as well as my 5K(14:49). But let's say I were to work on racing 100m- that really isn't going to do me much good, because it is outside of the range of the type of runner I am. If anything, it could hurt my training for long distance, as it is basically the opposite type of training required. Distance runners are primarily aerobic runners, and even a race like the mile is still a good portion of aerobic. Once you get towards 800 meters it begins to shift towards more anaerobic energy being used. In contrast, doing marathon training has helped me maximize my endurance for 15K-13.1 racing. This can trickle down to shorter distance runners as well. If the mile is your prime event, I believe it is good to do some 3K-5K racing to help strengthen your endurance end of things. But also, to do some 400 meter and 800 meter races to sharpen your speed/anaerobic energy required for the mile.
There are some really great marathoners I know that do some ultra distances, and I think those have helped their marathons because of the over-distance development. But, I do believe that maximizing some shorter distance times(could be 15K-13.1 for some people) is just as important for development. Some people doing the extra long stuff may need to go back to the shorter stuff eventually. It's constantly shifting back and forth. We just need to find what we need to work on for each period of time. For me, I think I have done a pretty good, consistent job working on the shorter stuff to support the longer races. I keep giving the marathon a try, and failing, but it actually HAS improved. And I keep getting faster at the shorter stuff, which I have improved in the most. Perhaps the next time I train for a marathon, doing some over-distance training runs of 30+ miles will be what I need to do, in order to race the 26.2 to my actual potential.