That seems odd to me. If you're saying that 7.5mph for 2 hours drains your battery to empty then I'd say you have a battery or charger issue. Or you're climbing mountains in Colorado with throttle only.
I'm in Texas on flat ground and weigh 150lbs but here's my stats on the 12.8. 19 miles @18mph avg all in eco used 2.9Ah, 17 miles @22mph avg all in #1 Boost used 4.3Ah, 17 miles @24mph avg all in #2 Boost used 5.3Ah, 9 miles @27mph avg all in S Boost used 4Ah, and 29 miles @21mph avg in 1/2 eco and 1/2 #1 Boost used 7Ah. By my estimate I should be able to ride ~80 miles at 18mph avg in all eco mode on a full battery. Hardly a toy to me.
Im saying that if you are 225 lbs looking to commute 20mi at power assist 2-3 in hilly terrain on a single charge, upgrade to the 17ah battery. I've given a real world account of usable miles with out motor cutout. Your theoretical stats are not very applicable to the proposition. Eco mode at 18mph is toy mode for the bike. Best for niche market