Any emergency room doctor will tell you that competitive sports lead to increased injuries. Sometimes the injuries are caused by outside forces, as when cyclists come in contact with traffic, or when golfers or climbers get hit by lightening. At other times, collisions with competitors are to blame. More often though, accidents and injuries are intrinsic to the practice of the sport and they happen even without contact with other competitors. Sometimes they happen without a distinct or well defined incident. We all heard of shoulder injuries in swimmers, tennis elbow, shin splints, iliotibial band, achilles tendonitis, and the like.
All activities can lead to injuries, but endurance sports are especially likely to cause overuse injury. Always ramp up slowly and prepare properly for any endurance event. However, be forewarned that even the most careful and best trained athletes do get injuries occasionally. And some injuries can cause permanent damage or even death. Deaths have occurred in all segments of ironman races. Deaths occur in swim races, marathons, and double centuries. Some popular prescriptions such as drinking a lot have given rise to deaths so be careful who you listen to and when.
Athletes can be more susceptible to infection, and certain cardiac ailments. Some will develop heart rhythm disorders or aggravate existing but subclinical ones. These can lead to sudden death. But perhaps the greatest danger to the heart comes from cardiomyopathy and cardiac hypertrophy with insufficiency. These conditions develop insidiously and are often only diagnosed when it is too late. It is hard to predict who is at risk, other than to look at family histories. Echocardiograms are of some use but early signs are often missed. Over time, genetic or other tests may become available, but it seems likely that all tests will do is give you some likelihood of developing the condition. It will still be up to you to decide what to do.
The bottom line is that endurance racing, as any other human endeavor entails risk. Checkups and tests can be done to minimize that risk, but none will ever eliminate it. Tests and diagnostic procedures produce false negatives, and occasionally people die who were given the all clear weeks earlier.
It is ultimately your decision and your responsibility. You should never engage in racing unless you are aware of and willing to accept the risks. Be aware that the more you push yourself, the riskier things get. Also be aware that everyone is different, and that some things may make one person stronger, while making another sicker. Statistics are good at predicting overall behavior but their application to individual cases is minimal and often misleading.
You have been warned.