One of my favorite discoveries about earning United miles is that you can get 450 miles every time you schedule and complete a Walmart grocery pickup order.
I always fill up my Walmart cart on my computer, but then I complete the order through the United MileagePlus X App on my phone. I often do Walmart grocery pickups once a week, which means I earn at least 450 miles each week!
We combined several different methods of earning United miles, and within about a year, both my husband and I were able to save over 60,000 miles each! The miles do not expire for 18 months, but each time I do another Walmart grocery pickup (or earn miles another way), the expiration date moves forward to 18 months from that date.
h/t
https://moneysavingmom.com/free-united-airlines-miles/