It also sounds like you have already done more than enough to strengthen your core. 5ks and triathlons are more vigorous than a Half, and can require a lot more raw strength from the core, than a marathon if run competitively. Longer endurance runs require as neutral a running posture as you can possibly muster, so all that strength may be unnecessary for these contests of balance and aerobic endurance.

I am particularly concerned that you mention two ab workouts per week. Overtrained abs are a major contributor to back pain because tight abs pull the rib cage forward. I would back off on those leading up to a long endurance event. The training required alone will be sufficient to acheive the balanced running posture you will need to avoid flare-ups during such a race. Remember that endurance running is a key component of core training for many athletes in other sports.  I agree with Damien that in this case, you want to begin to model your musculature for the target sport of endurance running. You have nothing to lose but potential pain.