This is so common it has a name among runners, and it is not detraining. Ten days is nowhere near long enough to lose fitness - meaningful loss takes weeks, not days.
What is actually happening is that you have spent months with a constant training load, and your body has adapted to that state. Remove the load and it takes time to settle: there is repair happening that was being suppressed, and there is a period where you feel worse before you feel better. Most people report the low point somewhere in the middle of the taper, and it very often resolves in the last few days.
The heavy legs and the bad sleep are both part of the same picture and both extremely common.
The thing not to do is the thing you are tempted by. Adding a long run now cannot improve your fitness, the adaptation from it would arrive after the race - and it can absolutely leave you tired on the day. Every extra session in the last ten days has a large downside and essentially no upside. This is the point where discipline is doing nothing.
One genuine caution: if you feel ill rather than heavy, that is different and worth treating as such. Heavy and flat is the taper. A sore throat is not.