No puff

I'm looking for a bit of advice.  I've noticed lately that my lungs aren't able to keep up with my legs.  

My legs aren't tiring over a 5K Parkrun anymore having trained up to the Great North Run this year but whilst they're definitely able to give me more I'm absolutely gasping when I get anywhere near a PB, currently 23:47.  I get a bit lost with all the different types of training sessions so can anyone point me at the type of session I should be doing so that I'm not running out of puff so quick?

Thanks, B