Discussion: Shall I work on "Building a Base"?

The orienteering season is winding down, with snow on the way. Throughout the autumn I have been emphasizing speed work in my training regimen. When not orienteering, I have been doing fartlek, hills and fast (for me!) XC running. The web running gurus seem to generally promote what they call "building a base". That is, doing a training phase of six or more weeks of longer runs at less that 75% MHR with very little speed work. Considering that I do a good amount of bike riding in that HR range, will committing to a "building a base" running training phase make me a faster orienteer?