My friend and i are planning a road trip from central PA to LA California, we planning on hiking, fishing etc... on the way there. What route would you recommend and where would you recommend to stop ? Thank you!

For hiking, aim for the 4 Corners area and expand the geographical circumference to include national parks and national monuments in northern New Mexico (Chaco Canyon, Bandelier), southwestern Colorado (Mesa Verde), southeastern Utah (Arches, Canyonlands), northern Arizona (Grand Canyon). I can't be more specific since I do not know how much time you are allocating for this trip.

Whose car and what month? Would you consider taking a train from Harrisburg to Pittsburgh and transferring to the Capitol Limited to Chicago?
Hang out in Chicago for several hours and then take the Southwest Chief making a 24 or 48 hour stop in Albuquerque.
I used to live in Central PA (Mifflinburg).

Have you looked into fishing seasons and licenses? Have no clue what they are out west but know they are strict here.

W/o knowing when (very important) and how many days no one can give you any useful advice.

I can't imagine taking the train for a fishing trip. Next he is going to recommend taking the train on a cruise.

I don't know where to fish in Los Angeles County.
If we take a cruise out of Seattle or Vancouver BC, we probably will take the train. My wife and I went to Newhall CA over Thanksgiving using Amtrak both ways. The return trip to Klamath Falls was on Amtrak Guest Rewards points.