Training program/advice for first marathon

So this has been on my bucket list for a while and I’m not getting any younger, and want to get this done next Spring/Summer. I’m 43 and in decent shape but haven’t run more than 5 miles in several years. Currently I run 3 days per week and run for time (2 30-minute runs and 1 45-minute run). I’ve trained for a marathon twice before and ended up injured on both occasions when my long run got to around 16 miles. Once it was shin splints, the other one was a hairline fracture. I don’t care about my time - I just want to be able to start healthy and finish. Any recommended programs or tips on avoiding injury while training? Thanks!

Cliffs: Help me train for a marathon without getting injured. All I want to do is finish.