how do you develop the strength to be able to hold a aero right angled forearm position?
It's all about core strength, not arm strength.  If you want to ride like this for long periods of time you should be supporting most of your weight with your core muscles - not your arms.  Your weight should be on your sit-bones, anchored on your saddle.  Then your back and abdominal muscles stabilize your pelvis and share the load with your arms.  Your shoulders should be fairly relaxed and your grip loose.

I've have good results from following Tom Danielson's 'Core Advantage' book.  It's specifically about core exercises for cyclists and has a stepped approach to take you from untrained to pretty fit.