Sorry for late reply. I did a last-minute 3/4 perimeter tour (9k miles in 4 months) many moons ago. They had not developed the ST yet so I headed home.
I think your 60mpd is fine but my biggest hassle was the weather. Due to me going on at with about 10 days notice, I started late in early July in San Diego to Vancouver to Maine to Key West. I left San Juan islands August 1st so I had to hook it to beat the snow in Maine. Then the cold chased me down south to southern Virginia.
If you route timing is not totally finalize, you may want to reconsider heading into AZ in late August/early September as the temps can be quite brutal. If time/money permit, at Missoula, go north and do some Canadian touring for a couple of weeks so you can not sweat to death and maybe make up some time going to Florida.
Either way, enjoy the ride!
Happy trails and may the wind be at your back!