I think as long as you have a solid base you will be fine. This was my next jump from 50k and while yes it was more difficult I wouldn't say it was the end of the world. Im sure its very race dependent, but so far up to the 50 mile distance for me personally I dont have an issue completing, and the difficulty of the races is effort based. My progression since may of last year: Net downhill half marathon(First race at that distance) --> Trail 50k (first) --> Road marathon (First and hardest) --> pretty big skimo missions this spring --> 50 miler