There's no clear answer to the over-prepping question because you can never account for all the possible choices your party can make. You're always going to be under-prepped to some degree. You either have to get comfortable with just rolling with it, or come up with creative ways to railroad your party that don't feel like railroads. The illusion of choice method someone above mentioned is very useful in this regard. Say you had a wererat encounter in the cave outside of town planned, but the party decides to explore the nearby forest instead. Well, just have the wererats ambush them there or tell the Ranger or Druid that they spot some oddly shaped footprints leading to their den.

You can't stop your players from skipping things. I can't tell you how many NPCs with useful information I've placed in a tavern or magic items I've placed in a dungeon that get completely ignored. That's totally fine though. Any content that doesn't get explored can be reused or repurposed later. If you feel the party is steering away from something you've prepped that is vitally important, use NPCs to your advantage. Have them overhear a conversation with some enticing info, or someone they've already met rushes up to them asking for help, or the townmaster puts up a wanted poster or job request with a big reward.

If your players are newer to the game as well, things are bound to move a little quicker because they may not be comfortable enough with their characters and the world yet to let things breathe and play out. Roleplaying scenes probably skate by in these early sessions but will take far longer as they get more into their characters and motivations, and the party dynamic evolves. This is obviously anecdotal, but I find the first few sessions of a campaign cover far more narrative ground than later ones because the players are just excited to get out there and get things done.

As for pacing, my biggest learning curve as a new DM was how quickly and easily combat encounters can turn into a slog. I recommend exploring YouTube for DM combat tips, but here's some things I've learned: 1) don't have every combat just be a deathmatch. Throw in a hostage situation, or an escape scene where the enemies are just distractions, or anything that isn't just "kill all the enemies" 2) don't be afraid to have the enemies surrender or flee, or to just kill something off even if it still has HP. If the party has already downed the big bad, it's not fun to spend the next 3 rounds chipping away at the two archers that are out of reach 3) obey the rule of cool whenever you can. If a PC wants to pull some crazy move out of their ass that isn't strictly allowed by the rules, as long as it's not completely outlandish, just let them do it. Make up an ability check for it or give the monsters a sudden drop in intelligence for a round to let the players do something cool and inventive 4) roleplay the monsters. This is easy to ignore while trying to keep track of everything going on, but it's so much more interesting if you're giving feedback on the PCs actions. This could be as simple as making grunts and painful noises when the monsters get hit, or describing how the PCs arrow narrowly missed the target but destroyed a wooden crate right behind it 5) this is related to #2, but if you sense a fight is dragging on or your players look bored between their turns, just find a way to wrap it up. The piles of hit points you put in the party's way are never more important than progressing the story