The odds always seem 'good', most people I speak to seem to regard 1 in 5 as a certainty.
If they are offering 15500 places in the ballot, a straight 1 in 8 chance, 25% of people who applied for 10 years would not get one place! Adjust up or down for either 1 in 5 or 1 in 10.
It is unlikely to be that simple though as you would actually want a spread of times for the runners and a truly random selection from all of the ballot applications could throw up some very perverse results. It is likely that race times of 3:30 - 5:00 hours are very heavily oversubscribed, even considering that this is where the median result turns up on race day. So I'm sure something more sophisticated than a random number generator is used.