Disclaimer: I'm not a great at any of the sports and didn't participate in either sport at any level when I was younger.
That said, I came into running after 6 years of fairly hard core cycling. In the 5000 to 6000 miles per year range, with a lot of those miles as fairly aggressive club rides.
I started running going into 2009 at age 35 just because I needed something to do in the winter, and wanted to maintain my cycling shape (go figure). Keep in mind I did not have a running background. My third run I did 22:30 5k (why did I race that quickly? Who knows). And yea it's a very sad time, but this is basically "off the couch" and in my mid-30s.
A couple months later I did a hilly 10k under 43, and was passing people on every hill. It was like the hills didn't even matter. I was still thinking 15 miles per week running was great. I was still cycling too, when the weather cooperated.
I went on to run a 1:27 half within a year, never really getting over 30 mpw but the two or three times I did a 14 mile long run.
Then what happened in 2010? I decided to quit cycling, and run only. I got up to closer to 40mpw but couldn't go higher because I kept getting injured. I started to gain weight. All of a sudden I'd have good days and bad days on hills.
Yea, I ran a couple more PRs in the half in spite of that, but it kept getting worse.
Going into 2013 and 2014, I had more bad days than good on hills, still gaining weight, get injured otherwise.
Now in 2016, I basically don't run and don't cycle. I run about 12 to 15 mpw and all hills no matter how slight are my enemy. Of course I've gained way too much weight so that doesn't help.
I think that cycling gave me great power for hills and rolling terrain, and was a great supplement for my aerobic systems and also ability to recover after hard efforts (during a run/race). On the flip side, once I removed all that "extra work", I couldn't make up the calorie difference with just running, and kept gaining weight. Yea, cut down on calories eaten then...but that's hard when you're used to a certain way of eating or living.