Chance of getting in is about 50%, but that will certainly decrease in future years due to new regs. Chance of finishing is less than 50% - doesn't matter whether you're fast or slow. For example, Scott Jurek has only finished once in five attempts (in 17th), and multiple MdS winner Mo Ahansal finished his only attempt in 125th place!
Despite being currently injured and having to miss this year's GUCR, I have a place in UTMB for the sixth year running. I've had 3 finishes and 2 abandons previously, and have been round the course in training another 4 or 5 times. Happy to help with any advice. Just DM me or ask on here.
I did a race report on how it beat me up last year on another thread, so I'll dig it out in case anyone finds it useful/interesting. Just to put it in context, I'm no elite, but I'm no beginner either. I've run 40 ultras in the past 10 years, including a few podiums a couple of wins and course records and I've a marathon PB of 2.45. Four weeks before last years race I went round the course on my own unsupported in three stages and took 28 hours. But the cumulative effect on the race was much harder than I anticipated, despite having done it before & knowing every inch of the racecourse...
...ah, here it is