I'm going to DM my second campaign as an experienced DM. I already have the main lore all planned and also the initial events that will (or maybe won't) happen.
I know it will be pure fire at the beginning but I fear that at some point in the middle it will get a little boring like my previous (actually current) campaign. In the current campaign, the pc's lost interest in the main subject of the mission and start craving more for side-quests, fillers and any piece of side-lore and combats other than the main point of the campaign.
In the next campaign, the PCs will start in a cloud of mystery, and the lore will unfold in pieces in front of them. At some point it will be all clarified and they will have to set a mission for themselves, and they'll probably do, so there will be a long way chasing their goal (and this is the part where it gets boring).
I want to know how to keep the main mission alive during a long period of distractions in the adventure? (traveling, side quests...). And i don't think i want to make the villain or his lieutenants visit the group multiple times or something like this. I want to know how to keep the pc's engaged during this mid-game long run. (i hope that it's specific enough now)
How do I keep the players' interest in the plot during an extended campaign that allows for exploration, Player Character freedom, and side quests?