In other words not only do you have to pass selection & recruit training, you would on average have to serve between 4 to 6 years before being eligible to apply, by which time the job may well have changed significantly.
The only service which offers direct entry as a PTI is the RAF, but there are very few places and the RAF open & close branches to recruiting. You would have to investigate further whether they are actually seeking to employ them at the moment.
The PTI's down at Lympstone are stupidly fit. When i had my look at life course down there one of our main supervisors was a PTI who runs for England apparently. Not sure on the name, but he's the guy that leads them through the endurance course on the PRMC thingy that Blue Peter did :p
i would have thought they would be stupidly fit, because in all respects they must be doing nothing but phyz every day due to the continuous 32 weeks training programmes that are always running. well thats lympston PTI's anyway i expect