Rough estimate for calories burned is 40 per mile biking and 100 per mile running a mile, with no elevation gain. That says about 2.5 to one ratio. Triathlons are more like a 4 to 1 ratio.
I used to run some and bike a lot more, a 6 mile run for me was a lot harder for me than a 25 mile bike ride when I first started running. My wife has run a half marathon and felt it was much harder than a metric century (about a 5:1 ratio), but she is more of a runner than a cyclist.
But for me, it is more about time than calories - hours of elevated heart rate. A 2 hour+ bike ride is where the magic starts to happen for me, at least fitness-wise. That is about a 30 mile ride. At my 9-10 minute per mile "running" pace, 2 hours would be 10 - 13 miles but I've never run that far! So, I'd say a 6 mile run in 1 hour is about equivalent for me to a 30 mile bike ride in 2 hours - closer to my wife's 5:1 ratio.
She finishes faster and gets to drink wine while I'm still on the bike drinking warm water from a dusty water bottle...