I've only recently been introduced to mileage runs and I've been really intrigued since I learned about them, but I can't seem to figure out how to save money in the long run with them.

For instance, suppose I'm flying UA with 1K status, and I want to book a round trip ticket to say, PVG, or somewhere. From what I can tell, the cheapest first-class round-trip ticket is 70k miles, and if these were earned on a mileage run at 3 cpm (+100% bonus from 1K), then that's $1050 + the flying time to buy that ticket, not to mention the expense of getting to 1K in the first place.

I understand that some people (including me) really enjoy flying, so the flying time might be negligible cost, but am I missing something here?

I feel like it would be more efficient to just buy the economy ticket, as even if you were doing it to travel in first class on your award ticket, you'd have to do all the mileage runs in economy anyways?