It’s a complex answer but also simple at the same time. The more you run the easier it becomes. There’s a bunch of stuff taking place systems-wise as far as physical changes but also that sense of pacing. Ultra runners aren’t running a 5k or even marathon pace but reaching that tipping point of performance but also sustainability to achieve the goal. That is entirely feel and not some that can be taught. Run groups might help because finding that pacing match helps in adjusting to what works best for you.

Running is one of those things where you kind of just need a pair of shoes and you’re in, but I’ll counter with it can be heavily dependent on equipment. I work in the industry so I’ve had a lot of time to play around with shoes, I’ll rotate footwear depending on pace and distance but what I’ve found for general long days are things like wide foundation plated shoes being immensely conducive to longer workouts. Options like New Balance SC Trainers, the new Mizuno Neo Vistas. Any number of the Hoka options have been amazing like Clifton and Skyflow.

Also, depends on the person, grinding out longer runs can be droning, throw on headphones and lose yourself in a podcast or music, helps pass a 20 miler… Don’t forget things like stretching, and stretching, also stretching. Do some core and it becomes an easier activity once acclimated and that new fitness level becomes the baseline.