Okay, I see it this way, 5 options:
1)if you join the RAF now, stay there 14 years, you'll be 40 when you get out. So you'd still have a chance of pursuing the PT career.
2) On the other hand, if you start your PT career now, and still want to pursue your RAF dream later, at the age of 36 your PT career would be brought to an end. (if you don't wish to start as trainer at 50)
3) Go with the RAF as your only career.
4) Go with the PT as your only career.
5) Choose something else
You'll have to make a decision eventually so try putting pros and cons on paper, maybe it helps you clear up your head.