Hello Wiseowl,
I am new to cycling after running 4 marathons. My husband and I just completed our first century in Lake Tahoe, CA in June 07. I have to say that both marathons and centuries are challenging in different ways. I found biking to be harder because of all of the mechanical things I needed to know: such as changing tires, shifting properly, throwing my chain, etc. You don't have those kinds of problems with running. For me, I've struggled with more aches and pains with my running and find it to be much harder on my body than riding. I took me 5-6 hours to do my marathons and 9 1/2 hours to complete our century (about 7 hours of ride time-7,000 feet of climbing). It took WAY longer to completed an endurance event on the bike!! The "plus" was that I was able to walk normally after 100 miles-not so after running 26.2miles. The "down side" was that sitting after a century was very painful for me!!! Like I said, they are challenging in different ways. To be honest, I'm kinda "hooked" on the bike, which I've found to be much more "forgiving" on my aging body. While my running days are not over, I'm spending more time on the bike (can also do it with my husband-couldn't get him to run if his life depended on it !!!). Anyway, good luck!! I've found that a running background can really enhance your cycling. And yes, you can "hit a wall" in both running and cycling!!!

Blessings!
Susan