Will it be harder for me to get into pa school if I go into the marines?

Remember that the Marines have their medical care delivered by Navy Corpsmen.

It helps to have medical experience when applying for PA programs.

The Tri-service PA school is in San Antonio. Perhaps you could attend and continue to get full pay and benefits as you attend the program!

I  attended in 1981.  Worked out well as a long career!

You'll need to look at the admissions requirements for the PA programs you are interested in. Many of them have time limits on how recent your prerequisites must be (e.g., degree completed in the last seven years; biostatistics within the last four years, etc.). A four-year enlistment could push you past the timeline for some programs.