26.2 -> 31

How hard the last 5 miles of a 50K are will depend on how hard you go for the first 26?  (Just like a marathon...) 

If you really go hard in the marathon, 4 weeks to be ready for the 50K will be about as close as you want to go.  I ran a 30-mile trail run 8 hours after finishing a marathon this past weekend.  In my case, since I knew I'd be doing them back to back, my strategy for the marathon was different than what we did in May (4:11 finish).  We targeted 5+ hours to finish the marathon and would walk every hill.  We were targeting an average 1:15 per loop (it was 4 loops) without the benefit of a watch or other timing device, and our loops were 1:19, 1:18, 1:18, and 1:21 (they did have a clock for the first loop because of the simultaneous 10K).  I'm pretty sure I could have run a 5th loop and been in the 1:25-1:30 range Friday night. 

I'm not sure why you think you'd be 2 min/mile slower than a marathon if you're running an additional 6 miles on a converted rail line.  McMillan suggests about 30 seconds/mile slower to run a marathon over a half-marathon and a similar 30 seconds/mile slower for the half-marathon over a 10K.  The pace between a 20-mile run and the marathon is about 10 seconds/mile, so doubling that or even being generous and adding 30 seconds/mile ought to give you enough to finish the 50K as long as you've trained well. 

Mentally, the last 10K is probably about the same whether you're talking a marathon or the 50K.  The 10K prior to the last 10K is (IMO) the big difference. 

Good luck!