In regards to a marathon, its going to mean basically anything from the range of 5K pace to lactate threshold. Technically, it could go as far as saying anything sub half-marathon pace, but this would be the most specific definition for elite athletes.
5K work is effectively v02 max work, which is probably the fastest they'd ever go. They no longer need that sub-60 pace work in their legs. This would be pretty minimal I'd assume.
10K pace work, or basically critical velocity, would probably be where they would spend a decent amount of their time. This would provide a decent amount of support for the faster paces, but would be more speed than strength.
Lactate threshold would probably be the blending point between speed and strength. They could spend enough time here that it would be fast for their marathon legs but provide strength for the marathon effort.