Running

Do shorter runs until you can run regularly without that pain, then build up. Your strength and technique will improve together, with any luck. Pushing through the pain is a great way to injure yourself.

Keen cyclists are often shocked at how crap they are at other athletic activities. Cycling only exercises a limited set of your muscles and doesn't even use your legs fully; running demands much more from you. It's much easier for a strong runner to become an active cyclist than the other way round, IME.

I agree - as it turns out, I'm using fewer muscles than I should even for cycling (due to missing some on my right hand side) so running regularly has involved teaching myself to use muscles that I never use, which is a lot of ouch at the outset.