Well done, onwards and upwards! A 400-mile or even 500-mile month should now be easily within your reach. I would like to have a 10-mile e/w commute, but the geography is against me (it's about 2 miles, and honestly I don't fancy a 10-mile loop just for the hell of it first thing in the morning! I go for a short spin most lunchtimes). With my back as it is (mostly OK, but complains sometimes) I'm satisfied with that.
Ironically, it was my reply to a thread with exactly the same title as yours (on that occasion proclaiming a first 100-mile single ride), on a different forum, that led to a sequence of events ending with things going tits-up for me vis-a-vis that forum. But this comment of mine is totally OT, sorry! - history won't repeat itself.