Just tell the landlord RIGHT NOW what happened and what you are doing about it. Then send in the late payment and the next month's payment in advance as a sign of good will.
Landlords don't like chasing money. It takes time and effort and fuel and all kinds of hassle. They want to know what is going on, and that it's a one-time thing.
But if you're tired of paying that much in rent, do something about it, too.
