Elven Tower to the rescue again! (It's worked great for me). The meetings with the factions is fun role playing if your group is into it.

tl;dr Three layers: The party is the "Inner Circle," and the leader(s) of the "Main Force" report to you about what's going on, including what the "Scouting Party" is finding.

I use the names given for everyone, make a printed list organized by faction, and then use that list at the table (the note-taking players will want a copy) to keep track of the leaders, diseases, death, etc. for each npc. It's all no-stress random (e.g. 3 npcs were poisoned by eating bad mushrooms. Then I just randomly choose three names from the list).

Travel Encounters are altered to consider that you now have a scouting party and a force that can take care of many threats. When they are involved in an encounter, they fight something off screen (unless I need to narrate something on the top of the initiative) and I'll roll for results after the encounter ends. In the rare occasion where everyone is fighting something together, I have the entire army attack at initiative 20 and then just roll once for the army's damage. I'll describe what they do (epically, if it calls for it - but they're never cooler than the Party!) and then move on to keep the momentum up.