If you restrict your gearing you'll still ride at the same intensity within the confines of that gearing.
If you want a guaranteed fix, get a heart rate monitor, wear it for a few rides between now and the DD, do a little reading on the subject then ride a few rides without a cycle computer and use your heart rate as a guide to effort.
If you can be bothered to observe the numbers, you'll notice a tipping point in effort vs average speed. I generally find that if I ride at an average of sub 155 beats per minute, I'll clip along at about 15mph average speed and be able to ride like that for hours. To hold an average of 17mph I'm probably at about 175bpm and I wouldnt want to do that for more than 2 or 3 hours. It sounds techy written down but it takes little to work it out and it's good to know.
With all that said, the DD promotes cruisy riding after the first 15 miles, even though it's flat for most of the 120 miles. Very few stomp it out at warp speed. We covered it in 9 hours last year with a couple of lengthy stops and finished tired but not destroyed.