I haven't done that either although it's a vital to do list item... easier younger/sooner than older/later!
Yes I reckon I have 5 years left or so. My best is only a pathetic 3.24 so there is room for improvement.
My main issue will be finding the time for midweek sessions, particularly the 90 minute Wednesday runs...
The other thing I’m keen on doing is the NYC Marathon. Conveniently they let you in on half marathon times too, and this year it’s 1.25 and then next year 1.28 (as I’ll be 45 by the 2021 marathon), which sound like good targets to aim for!