It's certainly doable as people do it every year. As long as you're physically fit (and I mean fit) you can do it. It's just not a lock like it is for NROTC. You should also look into the Platoon Leaders Class. They all provide a path to commission as a Marine Officer.You can apply for an NROTC scholarship when the time comes as a backup to getting an appointment to USNA. If NROTC is impossible for some reason then the Platoon Leaders Course could be a good backup to that.
Are there any known specific ways to become physically fit enough for marine selection? If not I will make up something myself.
I had lots of friends including a roommate go Marine Corps and, while fitness is certainly a requirement it is not "super-fitness" by any means. Later on, my second ship was an amphib with
lots of Marines aboard and I often worked out with the Marine Officers and they were fit but again, not more fit than I was.
To answer your question though, the run times and pull-ups, etc numbers are widely available online, work on getting a top score on them as that is how you'll be measured. As a High School Freshman, I'd recommend running Cross Country in the fall - the 5K that High School Cross Country runs is pretty close to the USMC distance. Add in a couple of sets of Pull Ups, Sit Ups per day and work to improve up to USMC numbers. After Cross Country season, shift to Indoor and then Outdoor Track and run the 5K (if offered in your state/conference) and 2 Mile if the 5K is not offered.