NW became my airline of choice when I took a job in St Paul in 1997 and I became a MM with the NW/DL merger in 2009. As I recall I had about 800,000 NW miles and 400,000 DL so the merger put me at about 1.2 MM. Hit 2 MM in 2014 and retired in 2017 so a lot less flying now and on my dime. I'm now at about 2.5 MM and the bump to comp Plat is very much appreciated!
One of the ways to build miles for me was AF premium economy tickets (A/S/W). AF had some great PE fares from the US connecting in Paris and on to Asia, return to Paris then to somewhere else in Europe and then back to the US for US$2200-2800 and they carried a 50, maybe 75% mileage premium. I think I did 1 a year for 4 years. Hong Kong and Copenhagen, HCMC and Prague, Japan and Kiev. And I would get op-uped on the CDG-Asia-CDG flights.