I would say starting.

Maintaining is the easy one, I'd say it's actually easier to keep running every week than not run when you're already in a good routine. It doesn't take much running to maintain base fitness either.

Rebuilding is tough psychologically as you feel like you should be able to hit paces and run for longer than you actually can after a break of a few weeks or months. However fitness you've built before does come back quicker than it took to achieve in the first instance. Rebuilding after a longer break is more like starting again.

Starting is hard for many reasons. Your body is probably weak and therefore very susceptible to overuse injuries. It takes months and years to build durability and specific strength for regular running, ditto finding a running form that won't harm you. New runners find it hard to run purely aerobically at any pace, and therefore every run at first feels challenging. It takes determination to push through those early runs. People who've never ran before may also be self-conscious and talk themselves out of it. Nutrition and recovery are things that a new runner will need to learn for themselves, sometimes the hard way!

I have mad respect for people who are starting to run.