It's possible, but only if you plan and prepare to spend 20 hours per day on the trail. This assumes that you're not already a distance runner and that you plan to walk the whole way with only occasional bursts of jogging.

I'm thinking of walking 10-15 hours a day

10 hours is impossible. 60 miles in 10 hours, average speed is 6 mph or 10 minutes per mile. That's the average pace for a marathon, impossible to maintain if you're walking the whole way.

15 hours is not impossible, but very improbable. 60 miles in 15 hours, average speed is 4 mph or 15 minutes per mile. Some people can walk at a 4 mph average for a long time, but most people can't. For an idea of how fast 4 mph is, the cut off for most marathons is 6.5 hours, average speed of 4.03 mph. Most people cannot finish a marathon without at least a bit of running. Averaging 4 mph over 60 miles would involve some running. I don't think I have to tell you that adding running, even a small portion, would be magnitudes more demanding than walking the whole way.

20 hours is possible. 60 miles in 20 hours, average speed is 3 mph or 20 minutes per mile. Many "very fit" people can comfortably average 3 mph for a very long time even past the point of exhaustion. You would need to carefully prepare and would need to get a lot of thing right during the event. Your nutrition, hydration, blister management, overall pain management, and sleep management would all have to be on point. It's possible. You would suffer. It would be terrible. You might not feel whole again for months after the event. But it's possible.

Honestly, I'd would advise beginning with a shorter event and working your way up. 180 miles in three days is a lot.