Most triathletes agree that cycling has some benefits for running not vice-versa.
Some of them thinks that of the muscles triathletes developed from his cycling will not get any faster in running.
Running is good (if we are talking about cyclist) prep. period overall fitness method as well other sports. That does not mean that cyclist should runn fast during recovery periods between races, because thye do nat have a much time to experiment with that, some of them do easy, recovery jogging instead of 1h recovery ride beetwen some races, it is individualy.
Traithletes in other way are very, very fast on bike as on running, so if you train hard it will not hurt at all, let take a look of 2008. Kona winner Craig Alexander he did his 180km hell bike course for 4:37 h which is average speed of how many 41km/h? wright, and he still complete his marathon portion for 2:45. It will be interesting to see what will happened if he did first running portion instead cycling.
It is a matter the amount of strength/power you need to generate to go fast or climb big hills. Running, even at a very fast pace, doesn't require sustained quad power, and demands more from the aerobic capacity.
I could run 5 min/km on marathons and sub-4min/km on 5ks, but my quads would get toasted on the bike (in duathlon) way before my aerobic capacity was taxed when I started.
I did last year this kind of duathlon 10km running-40km cycling-5km running, what i was discovered that i will never do it again for sure, in simple words my aerobic capacity and treshold power on bike portion where i almost die, was less then on 1st running portion, on second 5km portion i was really toasted, lack of aerobic endurance and treshold again, i guess.
Running uses more of muscles, cycling really demands a lot from mainly the quads, and this will generally be the limiting factor for accomplished runners who switch to bike - all those runner calves and hams aren't a big factor on the bike.
Of course, if you have both the quad power and aerobic capacity, you'll be super strong on the bike.
In short running will not hurt your cycling if you runn sometimes occasionally, neither will improve your cycling much either.