how to improve personal bests from 1K to marathon

Start now steadily increasing mileage and work on lactate threshold. Keep it to one marathon a year and the rest of year work on 5-10k stuff. Develop a weekly schedule that you can do over. Consistency and building upon the years and seasons before is the key to distance running success.

Start with 20m sprints. After two weeks move up to 50m, then 100, 150 and finally 200. 3 sessions of each distance over 2 weeks, 10 reps. Run hard but in control. These can be done as an add-on to your main workouts. Continue with 100's to 200's once a week for the rest of your career. Never neglect your speed again.

just wanted to add that my marathon training has not been really serious.. usually train 2-3 months (with a record max MPW of 50) and then just regular 3-4 times runs while not training (covering from 30 to 40 MPW). And my marathon progression was not that linear, for example 2nd was a 3:11, 3rd: 3:16, 4th: 3:19, 7th 3:09, 8th 3:01, 9th 3:13...

Thanks a lot for the quick feedback!

I have a 5K tomorrow (aiming for mid 18, though I have not trained for it, LOL:-)), a 10K next week and a half marathon in mid September, after that I am free for the rest of the year, no race planned until May/June, where I will try another Marathon. So I can start implementing your tips from September on wards:-)

Correction: only 7 reps of 150 and 4-5 reps of 200.