If you run 10km++ you can focus on three kinds of running:
Do 1 minute at a high pace (70% of max?) and then 30 secs of slow running. Then, increase your high pace with 0.2kmh. Do this until you fall off the threadmill. The session should be 30 minutes so don't start off with too high pace.
Longer runs with variations in pace just over and just under your lactate threshold pace. Let's say 10 minutes on each. Your lactate threshold should be around your half marathon pace, maybe a 25km run. And then your two paces should be 0.7kmh over and under this pace.
This is very important for pushing your threshold.
Long easy runs
Just relax on these sessions but make sure they are at least 90 minutes