I agree with you and feel your pain. I have a marathon PR of 2:23:06 yet I am struggling to run a 5k in under 16:00 at the momment. I did a 4x800, 4x400 workout on the track yesterday and worked hard to average 2:30 on the 800's and 71's on the 400's. 71's should be my pace for the 5000!
Anyway, I missed 170 days last year and it has been very damaging to my fitness. I think it would have been less damaging to my fitnes had I missed the 170 days in a row. However, I missed those days throughout the year so I never had any consistency.
It is getting better and thinks are looking up. I am going to start a base building phase on April 17th in preperation for a fall marathon. Either TCM or Chicago.
So for the momment I am just trying to get out of the door on a daily basis.