purpose of a taper is to recover from the high intensity workout, but not lose any of that fitness.
Key to tapering is dropping volume substantially. A two week taper is not too long, as long as you've been doing some killer workouts for a while and have a lot of recovery to do. The problem with most taper programs is that you drop your intensity. This is what gives you that sluggish feeling
When tapering for my "A" priority bike races i might go from 15 hours a week of training down to about 7, so a 50ish% reduction in volume but my intensity doesn't change much. I'm going out, hitting the intervals hard, and coming home. A workout is basically all on or all off.
I'd have a two week taper, so doing shortened volume, but keeping the intensity going. Then maybe 3 days before the event i'd take a day off, then the two days prior are just to "tighten the springs" introduce some tension back into the muscles, but not enough to cause any fatigue.
If your feeling sluggish, then try introducing some intensity back into the workouts, don't increase volume, but a couple of race pace intervals, or for a marathon, maybe 5K or 10K pace for a few minutes (2-3). then plenty of recovery between each interval, and do a few more. So maybe you have a grand total of 8-10 minutes at 10 K pace, so it's not that much fatigue you are introducing, but you're keeping your system used to doing the hard work.