Anything from marathon long runs, long tempo runs or hard fast speed workouts or maybe it might of been the environment you ran in.
For me its a long interval session on a hilly muddy xc course last winter which consisted of 6x1 mile efforts at a little faster than 10k effort with 1 minute recoveries and to finish off 10x100m sprints on steep hill which was more like crawling at the end of the session.