Not that the training is not easy but come on this isn't like we are going to Indoc at BUDS and then have 6 months of ruthless training with an additional 2 years of intense physical training on top of that. Running and some simple strengthening exercises would suit anyone just fine
The PT at OCS isn't particularly rigorous on paper, BUT...
Most people who showed up when I attended did not work out vigorously everyday, and even the ones that did didn't do the same freaking exercise for 30 minutes everyday. When you go into an environment where you not only increase your exercise frequency on a narrow selection of movements, but do it on 4 hours of sleep and have to scream at the top of your lungs on every rep while doing everything at max effort, you start to get soreness and nagging injuries. Those nagging injuries suck and you won't have time to recover until candio phase.
If your body is used to running 2 miles a day, doing 20 minutes worth of planks and 20 minutes worth of flutter kicks everyday, then yea maybe you won't get sore.
Every SEAL or EOD candidate I sent to OCS said their physical fitness routine regressed at OCS, so it sounds to me as if the you can identify the SEAL candidates as the ones that won't break a sweat during PT
When we had watches on later in training, I timed an RPT session. It was 3 whole minutes. Of course everyone was sweating, but those mini workout sessions seem longer than it is. Someone who is used to doing long distance runs and swims wouldn't find that difficult, and it's well below their athletic ability level. Additionally, morning PT is a relatively short run for someone going SEAL and the exercise routines don't work the whole body.
Anyone who is on a decent fitness program and takes it seriously will regress at OCS because of the narrow exercise selection, the limitation to strictly bodyweight calesthenics, and relatively short durations of work. It seemed like the PT at OCS was designed moreso to get candidates to ace the PRT, which isn't very difficult to do.