You're a winner if you go the complete 26.2 miles, even if you walk. A well-trained walker can probably be around 5 hours.
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It's also not unheard of for some speed walkers to beat a good chunk of the slower runners.

You'd normally be timed in the exact same way the runners are, which is usually but not always via some form of electronic chip or tag attached to your shoe. Yes, you should receive some form of finishing time, and likely split times at various points on the course.

It will likely be virtually impossible for you to be able to tell in all the finishing times how many "walkers" were ahead of you and behind you, but you will be able to compare by overall time/pace. Most who walk the course are competing far more with themselves, if with anything.

For what it's worth, clocks on the course will be based on gun time, not your individual chip time. Since walkers are supposed to start at the back of the race, in some events that can mean 10-20 minutes or more from when the gun goes off until you cross the starting line and truly start, but the gun time is already running. Though you should be able to receive an accurate chip time in final results at the end, if you want to know accurate time while on the course, you should time yourself with your own watch.

Anyway, you would need to look up an individual event's timing methods, and you can likely look up past years' results to see how much detail is presented. Unless it's a marathon that makes a clear distinction in starting times for walkers and runners, you'll likely be timed just as if you were a very slow runner when it comes to timing method, results posting, etc.

If you need more detail, it would help to know the individual race you're looking at.
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