An 18:30 5k definately makes a sub 3 attainable, but how close you are/will get depend alot on how youre trained.
Naturally, the marathon is in the other end of the scale of speed vs endurance. I am in a similar situation as you - I have still not run a sub 3 marathon, but think I should be able to right now!
(has been a few years since I’ve been able to race, but got my HM close to 1:21 high so that gives hope of a sub 3:)
As for the training, I dont think there is anything magical of one plan vs another. More important is you general approach to training, and what method you “go for” (iow what kind of training you choose to do). For marathon-traning, my (undeducated) views are:
- Do alot of volume if you can.
- Quality-work should be split in three categories:
Fast intervals - do f.ex 1k intervals faster than your marathon-pace. For your level (18:30 5k), I’d say do 6-8-10 x 1k intervals @ 3:45-3:50 pr k. Dont do them too hard! I am a firm believer in rather cutting back the speed 5 sec pr km, and be able to sustain a few more intervals, than going too fast. If youre injury-prone, be esepcially attentive to these - go slower.
Intervals at race-pace-ish: Tempo-runs, 2k-3k intervals where you run around marathon-pace (4:15 pr k). Some days you can go faster, some days settle for 4:15. I usually do 2k intervalls (3-4-5 x2k @ somewhere between 3:55 - 4:10-pace.
Quality long runs: and this is imo the most important ones: Do long runs with sustained periods of running close to your race-pace. I like doing 20-25-30k runs, where perhaps the last 10-15 og even 20k are close to marathon-pace. You dont have to run 4:15 - but try to get quality-periods of 10k + where you run somewhere between 4:10 and 4:25. These runs take alot out of you, so be well rested going in, and take it easy for 3-4 days after.
I will normally do 2 or 3 quality sessions pr week. If I did a hard quality-long-run, 2 is def enough for me. If the quality-long run is in the easier region (either under 10k of hard work, or a slower pace closer to 4:25 than race-pace), I can do to invervall-sessions + the long run. If the long run is hard, I only do 1 intervall-session.
All other running is fairly easy (anywhere from 4:40 - 5:30-pace, depending on feel)
FYI - I run 70-90 km a week.