For the less adventurous, there's a shuttle van between Mandalay Bay and the Courtyard South Las Vegas - which is on the "correct" side of I-15 at Russell Road, south of the In-N-Out.
Conceivably, you could shuttle from MB to the Courtyard; then hoof it a few blocks north to the In-N-Out; then gorge vociferously; then waddle back to the Courtyard; then take the shuttle back to MB.
I'm the adventurous sort so I'd be fine with hoofing it across Tropicana. You don't actually walk on the freeway you're walking on Tropicana. Speed limit is 45 on that road. If you actually look before crossing the exit ramps, you'll probably make it.
