As an ex Sapper myself I can vouch for the fact that anyone volunteering for Para first had to pass the 9 Sqn pre-para, which was daunting.
Not just anyone got to command 9 Sqn, it could be a career breaker. The Sapper para sqn would lead the entire Airborne Brigade into battle, no room for any kind of screw ups, only the very best leaders will do.
The kind of determination and qualities of endurance that it takes to get through para training would probably get an overweight fifty five year old through the annual test and leave some people half his age looking a bit sick.