If you're getting slower and have evidence to support that conclusion, you're overtrained. The usual evidence is having to grab a cog lower on a climb you've done many times and that not getting any better. Training should involve getting faster. If you're not getting faster, you're doing it wrong.
All that said, a reasonable taper for a next Saturday ride is:
Saturday: 60 miles, some hills, some hard work but not too much.
Sunday: Off
Monday: 1 hour on the trainer of light to moderate effort with maximum efforts of 4 x 30" X 4'.
Tuesday: Same thing, but 3 X 30".
Wednesday: Same as Tuesday.
Thursday: 30' recovery on the trainer.
Friday: Off.
There are many similar tapers, but this is the idea. Way less time, but still a little intensity, just not enough to tire a person. The use of the trainer is to prevent overdoing it.
Some long distance cyclists simply do nothing for the whole week, but I don't think that's optimal.