Does anyone think it wouldn't be to difficult to achieve 12.5 miles in 3 hours... So about 4 miles every hour??

I saw it on the video I posted about getting into the Australian special forces, and one of their tests was to walk 20 kilometers (12.5 miles) in 3 hours 15 minutes.... They had to do it with a 30 pound or 30 kg pack on, but i'm considering attempting it minus the pack...

Just wondering if you guys have ever actually timed yourself over a mile or two or whatever,,, and what do you think a reasonable time would be if you evened that over 12 miles.. i.e. obviously can't keep the same pace over 12 miles compared to 1 mile. So can't just multiply your 1 mile time by 12..