I’ve only DM’d 5e, so excuse my ignorance,

... Is the concept of an adventuring day new in that edition? Did PF1 have such a thing?
It's as old as D&D, in a practical or de-facto sense. It's been explicit since, at the very least, 3e (of which PF1 is a clone, so yes, totally needs a prescribed 'adventuring day' to resource-balance classes vs eachother and encounters).
Any game with significant 'daily' resources (and that can be daily or just about any other time-based recharge limit) has introduced a factor that makes the quantity and difficulty of challenges faced in-between those recharges a significant factor to balancing said challenges against the capabilities of the PCs (that is, making them actually challenging). If the game also has classes and gives classes different mixes/versatility/power of such resources, then class balance also becomes dependent on that same prescribed pacing.
People always go on about this resting problem. I tend to find it is ok in practice but maybe I run weird games.
Nothing about the below sounds weird...
Take my current campaign. I started with OOTA. While travelling through the under dark the one big encounter per day was pretty much the norm. When stopping off at one of the mini dungeons the whole dungeon was generally needed to be done at once so 3-5 encounters. And in other towns larger dungeons were found in some cases where more than one day might be needed to clear the whole.
IIRC, the prescribed day-length in 3.x (and, I assume PF is no different) was to average around 4 encounters/day. So your campaign has some single-encounter days that favor daily-resource-heavy classes, and some 3-5 encounter days that are more or less balanced. In theory, longer days could favor at-will-heavy classes. So, that averages, overall, below the prescribed length. I don't think that's at all unusual.
If you were running 5e, which expects 6-8 encounter days, and 2-3 short rests, you'd be falling well short.