^This.

The C25K exists because so many people have never really run and most people don't just decide one day to run several miles. People aren't fit enough or don't even know how to run right. Cycling is different. I think anyone could jump on a bike and go 25 miles. They may be sore, they will probably be slow but it's doable. If you go out and ride 5-10 miles for a couple weeks then 10-15 for a couple weeks, etc. it won't take more than a couple months to build up to a comfortable 25 miles or more. Building up to a fast 25 miles or a steep 25 miles now that's different but the same process applies.