Regarding cycling:
If you choose to race, it can be a very competitive sport.
Road racing is a very results focused sport. Winning is much more highly regarded than placing, which in turn is much more highly regarded than just participating.
Being at a race just to participate might not be all that much fun due to the results-oriented culture of the sport.
Depending on your abilities, you may be suited to different types of events within the sport. Cold, windy, flat races select different riders than do hot, hilly races. Track races are something else entirely.
Reformed runners usually do well, especially in hills.
Racing will attract competitive Type A alumni from other sports. There are local races whose participants have included former D1 runners, former D1 wrestlers, former D1 swimmers, NFL alumni, etc. Also making successful appearances are non-athletic-background-types who discover, as adults, untapped aerobic talent. There's also a fair representation of former military. The common denominators among participants seem to be competitive attitude and willingness to work.
Racing can also be time consuming. The successful local amateurs with real jobs frequently put 10 to 15 hours a week in the saddle, and, in some cases, up to 20 to 25 hours a week. Talent will allow adjustments to this training load.
Cyclocross also provides good racing opportunities, and it's pretty sociable, and it's a solid workout. Its crashes can be a bit more frequent, and are usually much less severe, than in road racing. The speeds are lower, the riders separate more, and the ground can be softer.
Mountain bike racing and events are different. There's still a competitive cohort at the top, with former runners showing well. But, the events are also fun for those just there to participate and complete the course.
Mountain bike riding can involve some early frustration with technical skills. Once those skills are mastered sufficiently, mountain bike riding can be one of the most entertaining ways to get a solid aerobic workout.