++ on the pain thing. A good thing to improve all your riding is doing long climbs as hard as you can do them and not blow up. Though occasional blow-ups are OK, too, because you have to learn where that is. By long climbs, I mean at least 10 minutes. And at least once/week, and multiple climbs per ride. If you see a hill, ride up it. Make that your standard.

The other thing that really helps is simply mileage. If you are riding 200 miles/week, you'll ride a century and wonder why people make such a fuss over something like that.

But to answer your question, it's a little tricky. Running does a lot more damage to your bod per unit time than biking can. And doing damage and then recovering from that damage is called training. So if the reason your biking is not so great is because you are time restricted, then yes, running might help your conditioning.

But since you say your running was unexpectedly easy after doing all that biking, then I'll hazard a guess that you are going better physically by biking than you were by running, so just bike more instead of taking time from biking to run.