Mark,
I am in a similar position mate. Have had that slight feeling of tightness when walking and I really noticed during the first mile of last night's run (four miles) and the first mile of last weekend's long run (10 miles). Gonna rest until Sunday, suggest you do the same.
I have taking ibuprofen and icing it for 20-30 mins each night. This is the best I can do, it has been making it feel better. Although I suspect it;s gonna be hurting me on Sunday when I start to build up the miles. Then again it might not appear, very difficult to call at this stage.
Take painkillers with you on Sunday. I have just accepted there's nothing I can do about it and am focusing on getting the job done. As Bear B.Hind says, your preparation is complete now. Rest up and give yourself a fighting chance.
Good luck with it.