I'm late to this thread but thought I would add my thoughts anyway.
Having (temporarily?) given up running due to injury I can honestly say that I have no regrets and that my running targets were (mostly) achieved. The way I see it the best targets are those which are 'mathematically pure' and once you have achieved that goal then the interim steps matter little. By mathematically pure I mean targets with 5 minute increments (10k) up to 10 minute increments (half-marathon) and 15 minute increments (marathon). As an example, running a sub-40 10k is hugely satisfying such that making the leap from 40.01 to 39.59 is far more satisfaying that going from 39.59 to 38.59. Similarly the sub 1:30 half and the sub 3:00 marathon have the same kudos.
Having always hated the marathon and having achieved a sub 40 10k and a sub 1:30 half I guess the only target I missed was the sub 60 10 mile (boo hiss)