Though I had been planning to do this for a while, I finally went out a couple of weeks ago and bought a bike. I bought a Giant Defy 2 which is a decent bike, not ridiculously expensive, but not cheap either. If I end up riding like crazy, I'll upgrade at some point later.
And, since I can't do anything small, I took it out the next day and rode 9 miles, and then rode 40 miles the day after. Who says you have to start slow?
It was amazing to me how little I needed to start out slow. We use similar muscles between running and biking, but I didn't have any pain after the 40 mile ride and in fact when out the next day and rode another 15.
As for cross training, I'm finding that I can ride one day and run the next without any pain or sluggishness. On the running side, I find that I'm running a bit faster. I can't tell you how much I needed something to break up the constant pounding on the road. I needed it to keep my body together, but I really needed it to keep me sane.
I find that I really look forward to rides and to runs as well because I'm not doing the same thing every dang day.