Leave truck set for a few hours and it's Super hard to start

Hello,

It sounds like you might have issue with the fuel pressure regulator.

Overtime the internals of the pressure regulator will get worn and can have issues similar to what you are experiencing.
It is possibly not sustaining proper amount of fuel pressure as you start the engine.
Test the fuel pressure and verify you are getting appropriate amount.

You should have 60-66 PSI of fuel pressure with the key On and engine Off.

Here is a helpful guide of how to check fuel pressure:

https://youtu.be/aITQclp7zIE

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.