Simple rule: never run hard more than three times per week. Use the rest of the week for easy running and do it often to offset the effects of the hard runs. You might be able to do a fourth hard run once in a while, or if you’re still young and full of growth hormones and getting quick recovery, but as a rule, three is the maximum.
But what do I mean by “running hard”? Obviously if you go to parkrun or a race, that’s running hard. If you do an interval or speedwork session, that’s running hard. If you do some kind of tempo or marathon pace workout that’s running hard. Anything that gets you breathing hard or sweating counts as running hard. Introducing a new type of session, e.g. a long run, counts as running hard because your body isn’t accustomed to the work involved. It’ll need time to recover from that.
When Paula Radclifffe was at the peak of her marathon training she worked to a nine day cycle with one workout every three days. It’s the workouts that get you faster but you need the recovery days to allow the body to rebuild. Slot in too many hard sessions in place of easy runs and you’re making it hard for the body to adapt.
Often though, what runners think of as easy sessions actually count as ‘running hard’. If you arrive home sweating, or your breathing is raised, or don’t feel like you could do it again, then it’s probable you’ve been running hard. It’s always better to err on the side of caution and run what feels like ‘too easy’. Generally the only runners who undertrain are those who don’t train regularly.
So that’s my simple rule – maximum of three workouts each week with the rest of the week spent recovering and building a base. By all means don’t even do three each week, Paula didn’t. I’ve seen myself and other runners make decent progress off just one hard run per week. But start to understand that a hard session is anything that’s more than “too easy” and start limiting yourself.