A few thoughts:
Give the adventurers a reason why they are in the adventure. Maybe they are hired or sent on the adventure by a ruler or cult leader. Maybe their community or cult is threatened by something. But give them something – that gives them a hook for roleplaying and it also gives them some engagement with the setting.
Let them learn something about Glorantha as a result of the adventure. Maybe they gain some insight into their cult and its deeds in the Godtime. Maybe they learn something about how their society (city, tribe, whatever) works. Maybe they learn a little bit about the power of the Empire of the Wyrms Friends or the God Learners. Maybe they see the consequences of some Gods War conflict played out in the world. But let them explore it rather than giving them a ton of text or exposition.
As a corollary to the above – let the players investigate! Be willing to borrow from Call of Cthulhu. Let them go to the library – if they have a LM character, let them use library use; otherwise, let them hire sages. Give them snippets of information which they get to put together and play around with.
Don’t try to attrit the adventurer’s resources like you do in DND! Two combats in a session are plenty! Heck, usually one (or even none) is enough. Let combat be something the players try to avoid as much as possible – alternatively, let them be creative and come up with a way to have combat on their terms. Short scenarios let the players use all their Rune points (which means the adventurers are likely much tougher in the combat); longer scenarios mean the players need to use their Rune points more sparingly, which means you can have more combats but need to be careful about overwhelming the adventurers.
Another thing to keep in mind. As a general rule, make your life as a GM easy and don’t give your faceless minions (broo 1, broo 2, broo 3, etc.) much or even any magic. Now that means that the adventurers can likely win even if outnumbered – but even then the players may choose to use magic points and Rune points to try to cut them down. And that’s totally fine – people using loads of Rune magic and big spirit magic spells should be able to defeat foes without. That is true to the setting.
But when they are encountering something truly significant – the dark troll priestess, the broo shaman, the sword master, whatever – throw the book at them. For example, give the priestess 8-10 Rune spells, her full CHA of spirit magic, an allied spirit, a few bound shades, other bound spirits (to give her an big pool of magic points), good armor, a few spell matrixes, and be willing to spend it all against the adventurers! Remember she’s got an allied spirit and some value trollkin keeping an eye out for trouble. By the time the adventurers get to her, she’s got a half-dozen spirit magic spells cast, she’s cast her defensive Rune spells, released her shades, and has held a few Rune points in reserve to cast some big Blinding spells (boosted with magic points) against whoever looks most dangerous. Also remember she’s got a bunch of faceless minions and maybe a bodyguard who is significant in his own right (he’s a great troll!).
Similarly, with the broo shaman, remember he’s got a bunch of bound spirits including disease spirits, a fetch, maybe an allied spirit, a load of spirit magic spells, and a fair number of Rune points. Hit those adventurers with Crack (break their non-magical weapons and best armor!) and Fumble. Use that allied spirit to add extra Chaotic features! And remember, the other broo will fight fanatically for their shaman – he holds the threat of Curse of Thed on them!
On the other hand, if you want to make it easier for your players, the have the significant bad guy hold on to more of their Rune points. Maybe they think this is only the first of several fights. As the GM, you have a lot of control over the pacing.
Something else – if you are writing a scenario for publication, remember there is no default party. The adventurers might consist of six veteran Humakti who treat your “near-impossible” combat as a cakewalk in the park. And then they find the basic investigation or social aspects a bridge too far.
Alternatively, the party might consist of Ernaldans, Lhankor Mhy scribes, a Pavis cultist, and a Lanbril mafiosa (true example!), and they find the “training combat” nearly impossible!
That just is how RQ works. It is ultimately up to the GM to determine whether a scenario is appropriate for their player characters, and to make adjustments as needed.