I see the loop you describe as having few, if any, intervals where a day's travel at 200-250 miles/day will not bring you close to most any sort of food, fuel, and supplies you may need. Certainly laying in a couple or three days of nonperishable backup supplies is a good idea, as is missing few opportunities to top off the fuel tank, but from recent personal experience your general route from Spokane back to where you'd get off of I-70 in eastern Utah should be "no problem-o" as to food, fuel, and supplies.
I think each has their own comfort level as to miles per day. I start to "hit the wall" in terms of ability to focus and react well after 650-700 miles of fairly open Midwestern or Western US Interstate. In the more congested East, I start falling off of my prime at 550-600. For off-highway traverses, it depends a lot on how challenging the route is. Valley floor graded gravel county, BLM, or Forest Service roads normally allow me to run 40-45 mph unless washboarding is bad. Routes ascending and descending ranges or within tight canyons are more demanding in terms of focus so the slower average travel speeds and low speed required of such routes may make 200 miles a stretch.
Case in point: In late July last year, we left the pavement west of Clark Canyon Reservoir (southwestern Montana) at around 11:00am. First segment was 55 miles of mostly graded gravel road ascending to a divide at about mile 30, then down the opposite drainage along a more remote road with some narrow canyon segments. That was followed by 30 miles of Interstate, then another 50 miles of gravel road. The second gravel segment was wide open and mostly level--no passes or canyon segments, but it was badly washboarded in places. On each segment we stopped often to take in the views and look at the Benchmark maps and other reference material such as Roadside Geology of Montana. It was 6:30 pm by the time we reached our destination and I was seriously ready to get out of the truck. We'd run about 150 miles of 2-lane state highways earlier in the morning that day for an overall 285 mile day. It was all we wanted, but it was a terrific day. Point being there are many variables in play--so many it's hard to make generalizations.
Foy