I'm a cyclist and runner (5ks) since I started cycling my running times have improved , Running help cycling?

Cycling minutes are not as efficient as running minutes. Running will get your heart rate up faster than cycling.

5,000 meter events aren't that strenuous. Its when you get into half marathons that test your body's ability to do a good pace throughout without running out of steam.

Cycling could help running in that aspect. But it would take a lot of cycling minutes to improve aerobic capacity, more than running minutes. The triathletes spend different times on bikes than running. They know the difference and their weaknesses.

I am a cyclist and runner (5k and 1/2 marathon) also

depending on your age and fitness level and genetics cycling can help in different ways. I'm in my 40s, ran a lot in my teens and 20s then switched to cycling for about 10 years, then got back into running to train for marathons but always broke down near 20, due to shin fracture issues for 3 years straight. Taking is slower this year for a spring marathon, but more than half of my endurance workout will be on the bike. I had no trouble running 30+ miles in my 20s, but age and genetics is catching up to me. Anytime you can raise your heart rate to an endurance level it will help, I plan on only running one long run every 10 days.

If you were not highly trained beforehand, then yes, any aerobic gains from cycling will spill over into running. If we are to belive Joe Friel, (and he has a pretty good rep)

taken from Joe's blog posts about specificity: Joe Friel's Blog

"One I deal with a lot has to do with triathletes and bike races… Many multisport athletes believe that bike road racing is good training for triathlon. It’s not. Bike races are, indeed, aerobic events, as are triathlons. But that’s where the similarity ends. The outcomes of bike races are determined by two-minute episodes when all hell breaks loose. They are anything but steady state aerobic. Bike racing has a huge anaerobic component which is critical to success. No one in their right mind races a triathlon that way. Triathlons are steady and anaerobic intensity is avoided. A bike race done by a triathlete is largely a wasted workout day. It’s even worse than that because the recovery after one of these delays when the next, truly specific triathlon workout can be done."

You should check them out, they explain much more than I care to copy in a single post :).

I am 64 and an Ironman finisher. I can swim, I can cycle, but due to bad knees I can only jog a bit but can speed walk at a 10mpm pace.

My bicycling, no racing, has allowed me to speed walk at that pace because my HR while walking approaches that of faster riding, the higher cadence while bicycling correlates to my rapid stride achieved and the endurance from long rides provides benefits for marathon distances.