I think when planning mileage for any trip, the two biggest factors that need to be considered are 1). What is your body conditioned for and 2). What is the purpose of your hike?
I recognize the need to estimate distance for resupply, etc., but I also believe if you are not used to multi-day backpacking trips (even if you do overnighters all the time or do dayhikes all the time with a daypack), then the focus needs to be on not killing your body, because it will take a toll very quickly when you're not used to getting up and hiking day after day with a loaded pack. Yup, you can probably do 20 miles your first day, but doing that even 3 days in a row can make you miserable if you're not conditioned for it. Who wants to come home from the trip they've been planning for months with the biggest memory being pain?
So if you don't backpack all of the time, my advice is to never plan a multi-day trip with any day being longer (mileage-wise) than you're used to going on day hikes, and preferably plan shorter days than you normally do on day hikes, as the recovery time your body needs from day to day is much different when you're carrying a loaded backpack. (age is a factor too...as a 22 year old guy will probably have faster recovery times than a 45 year old man)
The other thing I recommend is asking yourself what purpose this trip is serving for you. Is it to just get into the hiking zone and pound out the miles (some people really enjoy this) or do you envision yourself taking lots of break and really observing/enjoying scenery, stopping and talking to people, stopping to take lots of pictures, etc.? Your mileage planning should really be more about being kind to your body as well as honoring the true kind of hiker you are (constantly moving type or frequently stopping type) while considering your resupply needs which aren't really an issue in SNP.