I'll also add - cycling definitely helps my running, but it doesn't get me to run-PR status. Specificty is still king.

However, I can run at 95% of my all-time peak PR speed with 33% or even as low as 25% of typical run volume as long as I'm cycling a lot on top. This holds for all distance from half-marathon to 5k. (Sorry, the marathon is one where one will REALLY suffer if you're cutting the run volume out for cycling. You really can't fake racing a marathon.)