I've been DMing for my cousin recently. He's been on a long overland trip. Some times random encounters end up being a lot of fun. Some times they're a lot of work for nothing. The last two encounters were brigands and pilgrims. There's a ton of work putting those encounters together, even if you don't get down to the nitty gritty details. So I put the brigand encounter together. Maylin the Wizard and his henchmen surprise them, and avoid them. The next encounter is (evil) pilgrims. Another chore just getting the composition of the pilgrims set. They're on foot. No surprise, encounter distance is 150 yards. "We ride the other way". But at least I have Brigands and Pilgrims encounters ready for the next time I roll them up.
I have a binder with several large encounters similar to that already rolled up and detailed... merchants, brigands, pilgrims, patrols, etc. As you say, they're too complex to roll up on the spot. It reminds me of the story Rob used to tell about how EGG would deliberately seek out bandit encounters because they had one of the best treasure types.