First Time DM need some advice for my first session

Hi all! I've been a fan of DnD for about 6 years and a player for 3 years. I've finished up my last campaign a few months ago and I volunteered to be the DM for the next one. I have a rough idea of how I want to start off the campaign. I plan for the party to start off at level 3 as that's how our group has always played. For the plot I wanted to have the party do a guard mission for an NPC who will be integral to the overarching plot, they'll be going from a coastal village all the way to the capital with some twist and turns along the way. but I'm unsure when to introduce these detours/subplots. Like when should I introduce combat encounters like would it feel rushed to have combat in the very first session. We also might be having a first time player joining us so I'm trying to be mindful about that. I'm planning to have this like travel arc to be the first arc in the story so I'm unsure of how many session to plan it for, we do like 3 hours a week. Sorry for all the rambling but I'd greatly appreciate any and all help, as well as any recommended resources for DMs, Thanks <3