Method 1 - minutes per mile
On a long run (more than 10 miles) I try to achieve 12 minutes per mile so that gives me a time of 240 minutes or 4 hours
Method 2 - Double last years winning time
Last year the winning time was 2:24 so that is a time of 4:48 for me. This is probably more realistic but unfortunately gives some indication of the terrain to be expected.
Method 3 - 90th percentile
This feels like the most depressing method, by what time have 90 percent of the competitors already finished. Look at the results from last year again this gives me 4:51.
So from all this I think my target should be 5 hours.