It is definitely NOT walkable from the Sands.
Several times we have added the $30 to our Cancun Valet transportation to go to WalMart on the way, and several times we have taken the bus instead.
I think it's a toss-up whether it's worth going.
On the plus side, the prices are less than at the resort mini-market, and there is a huge selection. If you are planning to eat in a lot, if you have a bunch of adolescents, if you buy alcohol, you can save. You can't get fresh meat or seafood at the Royals' Minimarkets, so for "real" cooking, WalMart (or Costco) lets you buy them. The Walmart bakery is amazing and really inexpensive. Beverages are definitely less expensive and there's a big variety. It's fun to see the different types of seafood, produce, and other foods, and also fun to go over to the rest of the store to see the Mexican versions of things. Lotteria is a kind of Mexican Bingo that is played daily at the resort, and you can buy a lotteria game to take home. You can get pool toys and innertubes for the kids. There is a pharmacy. If you do it on a bus rather than on the way there, you aren't constrained to just an hour. There's a McDonald's right inside of the store.
On the minus side, it's a half-day of vacation. We're experienced with the Cancun WalMart, and last time we went with a list and divvied it up among four people with four carts, and still it took us more than an hour. Later we compared prices with the minimarket and though most things were less at WalMart, some weren't much less, and some were actually more. If you plan to eat out or just do light cooking (e.g. just breakfasts/coffees/snacks), or if there are just two people, then it may not be worth the time.
After ~20 years of going to the Royals every year, we skip the WalMart trip and instead bring a few things that are harder to find and/or strange and/or way way over priced, and just stay on site. (Everyone has their own list - ours is Peanut Butter, dry chicken gravy mix, and microwave popcorn.) We either eat at one of the restaurants on-site (usually at the Tri-Royals rather than the Sands) or we get ribs or rotisserie chicken, soup, salad, Mexican rice, and a brownie pie from the Express take-out that is inside the resort restaurant. Leftovers from that make a second dinner, for example we'll make fajitas with some of the chicken, tortillas and beans and salsa etc from the minimarket.
For us, WalMart is now a rainy-afternoon activity if that, but it is fun the first few times if you're interested in seeing the Mexican version of things.
