Yeah, that's my experience as well going from high F to high A or B where the regional flights leave. But there are moving walkways throughout G & C, the G-C connector, and the tram from the start of C to start of A & B. Plus there are carts that might give you a lift. I've shuttled my family across the airport with 45-60 minute connections without too much difficulty, but that's usually to a smaller regional jet where I want to be the last person to get on the plane.

Given the current construction and congestion in SEA, I'd choose a 45 minute connection in MSP over a 55 minute connection in SEA.

You could also probably get to SLC on Delta if you missed the BOI flight - there might be a later connection between those two city pairs.