When I was into running, one of the more well respected coaches/authors emphasized a funnel approach. If you wanted to run a marathon, you'd still do things like 400m/800m intervals along with your slower distance runs at first. As you got closer to the event, you'd focus more on the distance. Conversely, if you were more focused on 5ks, you'd start out similarly, but limit the distance work towards the end.
The difference with running is that some people seem to have an ability to get fast by just doing easy runs - because there really is no such thing. In cycling, just easy riding won't get you anywhere (genetically blessed excluded).