Your second question is the million dollar one and could elicit a lot of opinions. I would not consider 30-60 seconds per mile "of" (I assume you meant "off") marathon pace as tempo running which I understand is sort of fast, faster than easy running at least. It's your marathon pace, even in an actual marathon there's almost no hard breathing. "Steady" is pretty self defining, pretty much the same pace or effort or close to it for your entire run. You can run steady fast, steady moderate, steady easy. I'd characterize what you say you're doing as steady easy, maybe sometimes steady moderate bit I wouldn't call it tempo for the reason I gave. That said, you can find examples of people who run very well on nothing or almost nothing but steady/easy. Check out Joe Henderson's LSD
If you want an example of what I'd consider steady tempo I'd look at people like Clarke, Clayton, Nenow, for almost all of his career, Jack Farrell's runners, etc. Clarke did what we might call steady progressions runs. He ran with a group around a horse race track, all starting together at an easy pace and would pick up the speed as the run progressed. Guys would drop back as the pace got too hard. Ron would usually be under 5:00 pace over the last 2-3 miles. So he was running under his marathon pace, about 5:20, for much of the run and wasn't too far off of 10,000 pace for some of it.
So yes, it can work. The entire group that he ran with trained like this and a few of them made national teams and two of them other than Ron got to the Olympics. It's what I did to get myself from 4:34 to 2:35 in the marathon and it was how most of the guys in my club trained, at least five of whom got into at least one Olympic Trial Marathon.
It's fallen out of favor in recent years. I can't say exactly why but I suspect the fascination with exercise physiology and it's emphasis on hard day/easy day running and the importance of intervals is a factor. And coaches really don't have much use for it because it doesn't leave them much to do.