Are prolonged interstate speeds (78-79mph) hard on small 1.5L engine?

Actually, wind resistance increases with the cube of speed, an even steeper exponential.
A couple of years ago I went on a looong road trip, 5700 miles.
Going across S Dakota and other empty spaces at 85mph I got 31mpg on several tanks.
OTOH when I vacation on Maryland’s eastern shore never going over 60, mostly 50-55, I’ve gotten 37mpg (with no AC use) in my Toyota Matrix.

Actually, wind resistance (the drag force) increases with the square of speed and the power to overcome that resistance increases with the cube. Also, the exponential function is different than the square or cube functions:

More info about the Fit.
Actually your small 1.5 l engine is large, because in the EU the Fit under the name of Jazz sold either with a 1.2 or 1.4 l engine. Speed limits on freeways in general higher in the EU so you will have no problem with your Fit here.
The 0w20 oil that is in your engine is synthetic, I do not think you can buy conventional oil with that specification.
Check you oil regularly and follow the MM.
I can recommend this because I am driving a Fit as well and with a 65% highway 35% city driving my change intervals around 7500 miles. Now with your serious highway driving I am not surprised if the MM let you go 10k or up.
Last suggestion: maybe leave 10 minutes earlier, drive little slower and then you will not get a speeding ticket plus certainly get better mileage as well.

The Prius gets its greatest advantage in mpg in stop and go driving, but even in highway driving it is more efficient than competitors. In thousand mile trips, almost all freeway miles at 65-70 mph we consistently got 50 mpg without any attempt at hypermiling. The only tanks where we saw noticeably less were with higher speeds and heavy air conditioning use. In other compact cars I’ve made that same thousand mile trip and only seen 40 mpg. The Prius engine is smaller than those of many competitors and is designed for maximum efficiency at the expense of power. And the car has excellent aerodynamics, critical for highway fuel economy. If I had a long commute I’d seriously consider a Prius.

True, the Prius does well, but the $ savings are much less going from 40 to 50 mpgs. Even at 20,000 miles/yr it is 100 gallons a year, not enough to pay the difference.

@HWYHONDA sorry so late getting back.
as far as the road salt goes, my wife is from PA and she had it for a few years before we met. I doubt she ever once cleaned the salt off the poor Hyundai. Honestly, knowing how she treats her current vehicles, I can say for a fact that she didn’t. I am from Louisiana, so salt damage was uncommon for me. I’m not certain, but I did think that the rust on her Hyundai hardware was somewhat premature. I had the feeling that perhaps Hyundai was using sub-par grades of steel for their bolts of something. I’m not metallurgist, not it was bad for only being a few years old.