7 km.
You need to walk about 7 kms to burn 500 calories. But it depends on the pace you are walking and your current weight.
Alternatively you would need to walk 5 km to use up 350 calories (a small meal or average breakfast).
Yes, this is quite an effort. It demonstrates calorie intake reduction is far, far important that exercise to lose weight.
If you walk at a leisurely 3.5 miles (5.6 km) per hour, you will burn 500 calories in roughly 90 minutes.