Running is about ABC
Arms - make sure you use them. Especially on climbs and keep your shoulders relaxed
Breathing - Breath in for two strides out on the third. Once you get your breathing sorted running gets easier(ish)
Cadence - Don't try striding out if you’re new to running. Think about moving your legs quicker rather than longer. After the bike leg you will run like this anyway!
As per Wilko - build up slowly. AND the best way to improve quickly and have fun training is to join a local tri club. I hated running before I got into tri. I am not fast by any means but over the winter I have run 6 off road 13+ mile training runs, all with my club (Rugby Tri)
Best of luck with the training