A few years ago I was chasing the goal of 400 miles in an annual 24 hour race. Working toward my fourth year, I took spin class in the winter to maintain. I went in thinking I was pretty fit, and staggered away from the first class realizing otherwise. Kicked my butt. When spring came and I hit the road, I was in the best condition of my life. I reached my 400 mile goal that year, and I believe the fitness I gained from winter spinning made the difference.
I expect a structured training program, if adhered to, would have the same or better results. I found having someone scream at me to get out of the saddle and push, push, push provided motivation to suffer that I don't have on my own.