I sort of went the other way. I did 4:49 this year and put 4:20 - 4:29 as my estimate as I hope to improve. Us over 50's have to stick together.
Norry,
If I get in next year and if the weather is kind to us, I would like to believe that I could get under 4:40, however there must be a point at which personal upward performance curve does ceases the age ceiling process starts to pull you back.
Hopefully I have not got there yet!