Training Schedule Adjustment

I have decided to not participate in the Salem Lake 30K trail race this weekend as I had originally planned to do. At the time I created my training schedule for the ironman triathlon the 30K run seemed to fit in pretty well, but my peak running distances are not quite as high as I thought they would be at this part of the schedule. Trying to do an 18.6 mile run would be too big of an increase from the 13 to 14 mile runs that my legs are used to right now, so to avoid potential injury I'm changing this part of the schedule. I initially thought that I could do a regular marathon training schedule and add the bike and swim training to it in order to prepare for the ironman. From trial and error, and from some of the things I've read about ironman training, it's not absolutely necessary to get up to 20 mile runs prior to the race. In fact, I feel like I would increase the injury risk and lessen the quality of my bike and swim sessions to the point that my body doesn't have adequate time to recover and be fully prepared to complete the race strong. I've got 6 weeks to go and I want to make sure I'm ready, but more importantly I need to make sure I can make it to the start line in one piece.