I used to be a runner, but at 39 years of age, have just switched to cycling since Christmas.
I've run 3 marathons, countless halfs, 10ks and cross countries, none of which particularly fast (3h58 fasted mara and 1h41 fastest half) but the years of running 3 to 5 nights a week caught up with me with back and knee problems.
18months of nothing led to me me buying a bike in March, and getting into cycling. I've found i'm just as much a plodder on the bike as i was a runner. It's a different feeling though. With running, there is no let up. Sure you can slow down, but there is no relaxing or coasting. Cycling on the otherhand, i think is about peaks of exertion. A hard slog up a hill and a fast free wheel down the other side to get your breath back.
My current long rides are 30miles ish, which is just a short warm up for most on here, but i'm feeling my way into it and looking to build this up slowly.
If in cycling i can find anything as rewarding as finishing my first marathon, i'll be very happy.
I wish i could combine the two sports but alas i can't as much run to the bottom of the road without back pain.