Here's a set of tables for race performance prediction. The data is taken from various formulae which were derived from world and age group records. It is applied here with a plain multiplying factor based on your 10k time. It is accurate to the extent that you must assume that in order to perform at each successive level you must be properly trained for that distance. You could be running your all time best 5k for instance and yet have far too little endurance training to even complete the marathon. Or you could be running well at the marathon distance but not have your optimal leg speed for races under 5 miles.

So look for the time corresponding to your last road race in these tables and you'll know, plus or minus a little bit, how well you should be able to perform at another distance PROVIDED YOU ARE JUST AS WELL TRAINED for the other distance.