This is a very interesting thread, always interesting to make comparisons between cycling and running, even if they are less similar then we might hope.
We often want to know what sort of workouts or performance levels equate to in the other sport. Based on little more than my own personal experience and a few casual observances it seems the converting athlete will experience the most success if they are a 800-Mile-(maybe)3k runner converting to a cyclist. Runners will have a fine aerobic base but there is such a huge demand for power as a cyclist if you want to be worth anything (i.e. competitive not just good at logging kilometers). Cyclist to runner can be difficult, aerobic base is there and so is the ability to suffer, but the body is often not ready. As others have mentioned runners need much more than a strong posterior chain and good endurance. In addition to all the stabilizers etc a runner will need good form, flexibilty, and a sufficient amount of snap in the muscles (different from power). I have also noticed hanging around club rides and club runs, that older runners are much more durable and sprightly than older cyclists, who seem to creak around before gingerly mounting the bike. Maybe this is because if you can be old and still run you are probably doing something right. If it is not clear, I am not a phsyio and am certainly not informed about any of this.
However the other comparison between cycling and running that interests me most is the reception by people on the street. I lived in a mid-large city in the northeast, kind of beat down area. If I was running, summer or winter, tights or split shorts, fast or slow, I would always get such positive feedback from the people on the sidewalk. Women would hit on me, men would cheer me on, kids stop and watch. It was all very good for the ego. Now when I would get all kitted up and go ride the same roads on the bike, I would get mostly the opposite reaction, confused laughter, insults from children, etc. Although the ladies still liked me on the bike. The average person just does not have much respect for someone on a bike, while it seems that running evokes some sort of capitalist, bootstrap-pulling, self-made man ideals about grit, success, etc. Many here and elsewhere echo this when we talk of the work, of lacing up some shoes and hitting the road. Don't we all love the blue collar runner, but in America we do not have the blue collar cyclist. Maybe in Belgium, France, or Colombia.