Hip Flexor weakness

Hello all, 

I’m a very beginner level runner and training for the Honolulu marathon in December. I started training about two weeks ago but had to shut down a little bit because of low back pain that got pretty bad. I’m thinking it’s from really weak hip flexors so I have two questions:

1) I still want to train, but not sure how to supplement since I know riding the bike and elliptical are not the same

2) any suggestions on preventing the low back pain coming back? (Back pain comes from sacral area that radiates out to glutes, maybe glutes aren’t activating like they should either?). 

Very beginner here and trying to find a sustainable training pattern that won’t force me to shut down after a few weeks of running.