Affiliate Disclosure: All links earn commissionReading Time: < 1 minute
http://www.runningforfitness.org/ has some calculators where you can work out just that. not sure how accurate they are. 1k intervals are what you need to improve your 5k time..as you are doing the marathon training i assume your distance stuff is sorted so you need 5k speed work. If it’s any consolation it doesn’t get any easier to get your PB down below where you are now!