What? No. The average in your scenario would be 20mph. If you go 10mph for 1 minute you’ve gone 1/6th of a mile. If you go 30mph for 1 minute you’ve gone 1/2 of a mile. Add those up and you’ve gone .666_ miles in 2 minutes, which works out to .333_ miles in 1 minute. .333*60=19.98. The extra .02 is accounted for in the decimal error.
I said 10 mph for one mile, not for one minute, and 30 mph for the second mile, not for the second minute.
The first mile consumes six minutes of time, the second mile consumes two minutes of time for a trip total of eight minutes. Two miles in eght minutes is 15 mph.
Ah, ok. I misread you. Apologies, and carry on. 
Generally you buy the cheapest gas the car runs well on. If it says regular in the book, you use regular. The heat contenct is about the same for all gasolines. Only cars with high compression ratios and/or turbos need premium gas.
If, due to higher altitude you can use a less expensive gas, go for it. It won’t affect your gas mileage.
My experience indicates you get better gas mileage at the higher elevations, especially with EFI.