To taper or not to taper...

My longest run thus-far is a hair over 18 miles. I feel I need to run a 20-plus one before the big day, but do I have time to do that AND taper? I dunno. So, I am thinking about running 20 on FRI (15 days till), and then tapering from that. I guess that's a mini-taper, right? BUT, I have a 5K to run the day after (SAT, 15th). I'd be in no shape to run 5k after a a 20-miler. Maybe run the 5 in in the AM, and then a 17-miler that afternoon? Still deciding...