I used to do running so I had a look at
the current minimum Navy Physical Screening Test Run requirements (the last row in the table at the bottom of that document)
which at first glance looked soft (for example, corresponding roughly to 13 minutes for three kilometers) but then I realized the candidates would first take a swim followed by three workouts and would run afterwards ... which is tough, I guess
The physical test immediately drops roughly 50% of the candidates.
SEALs is higher still.. Only 20% make it through BUDS.
That is some rough stuff!