I do a decent amount of cross-training on the bike and swimming and it helped me get from sedentary to a 20 min 5k and also helps me now to improve my aerobic system as I run more often.
However, out of all the elite triathletes, no one has really set the running world on fire yet. Alistair Brownlee, Olympic Champion in the Olympic Triathlon, only has a 10k PR of 28:33, and surprisingly he also split a 28:50 in a triathlon. He can barely go faster in a dedicated 10k road race than at the end of a triathlon.
Elite runners run in the 27 minute range, very elite around 27 flat. Those triathletes do more training than the runners, they do 30 hours a week for aerobic conditioning alone! Ironman pros even more, upwards of 40 hours and more. They also do more strength training etc than runners.
Gwen Jorgensen failed her marathon debut, being beat by some amateur runners despite training 40 hour weeks for her entire life.
Armstrong only ran a 3 hour marathon despite having one of the best developed lactate thresholds and VO2MAX in the world (according to his initial fitness test he should have been able to run sub 2:20 in his first marathon), and barely improved it to the 2:4x range which is still pedestrian and unbelievable for someone who spent his whole life conditioning his aerobic system.
Is it because cycling and swimming excessively gives them muscles that are very detrimental to running at a high level? Or what specific adaption of the human body are they missing that is necessary to run very fast 10k times or marathons that these triathletes don't seem to get.