There are four core components you need to work on to build your foundation for a strong run especially in triathlon.
Stability- work on strength though your core, hips and leg especially lower leg
Coordination- ensure your hip extensors and flexors are working in synergy and the timing is correct
Balance- develop a strong feel for balance to ensure you are as efficient as possible
Timing- all your muscle fire but do they do so at the right time and for long enough
Work on these during runs and with targeted functional training and drills and you will find improvements in form and efficiency especially over longer distances.