Lifting during 1/2 Marathon Training

SBRRepeat

So, background: I've been competing in triathlons all season, just at Super Sprint or Sprint distances but I want to start increasing my distances to Olympic and, ultimately, 1/2 Iron races for next year. I'm confident in the swim and bike, the run is my weakest sport. With that in mind, to motivate myself I signed up for a half marathon in mid-October.
All season I've been doing some light weight training, usually 2x a week and I've recently incorporated more compound lifts and stopped using the isolation machines entirely. I'm a few weeks into Stronglifts 5x5.
My question for all of you, though, is if I should be doing progressive heavy lifting while also training for the 1/2 marathon, or if I should focus on bodyweight exercises (I already do yoga/pilates 1-2x a week) during training and save the progressive heavy lifting for the "off season." I have a history of back problems and the strength training has obviously helped with that but recently, my quads and glutes feel like they tire out more quickly during bike and run workouts.

Hope that makes sense. I'm a little confused and any help would be appreciated!