Quick update - I made it! Time from the Alamo to the Texas Capitol = 18:46:20
Distance measured on MapMyRun for my exact route (including a couple small detours like to Buc-ees) = 79.4 miles
Spot tracker was running, and I followed the route I had planned. It all worked pretty well - I'll do a write-up with more details. After I sleep some more...
4/19/17 - report complete: