The quick and easy answer to this is the cliche “you’ll never know until you try”, and I mostly agree with that! Your body can do amazing things when it is challenged, and thus it’s important and okay to push outside of your comfort zone. That all being said, there is a certainly a base level of fitness and comfort that you should have when choosing what distance you challenge yourself with. The transition from a half to a full marathon can occur over a three month time period with careful training. When transitioning to an ultra distance, your training window needs to be longer (16-20 weeks at least). Having a good base of 500 miles under your belt will help you prepare for an ultra distance. This doesn’t have to occur over a specific time period, but should happen within a consistent running routine over a couple of years. Logging miles like that over a multi year time period allows your body to adapt to loading better (minimize injury risk) and transitions you from novice to intermediate or even advanced. This means you know your body well enough to know what you can and cannot do. If you’ve been running for several years and have your base, then the next question is what level do you feel comfortable with? Okay, now move one distance up from that, master it, and build each year to something slightly outside of your wheel house. Set yourself up for success, not failure!