26.2 to 50k?

Give it a try! If you got through a marathon a few weeks prior without huge soreness issues, you shouldn't have any major problem with the 50k. I'd suggest starting at a slower pace than your marathon and seeing what happens. If you walk some, so what? You'll still have a great time. All 50k races will have a lot of middle and back of the pack people walking at least some of the time. 

Most 50k's will have a really different feel than one of the mega marathons.......a much more supportive and friendly environment. I bet you'll have a blast. After my first trail 50k, I realized I'd MUCH rather do these smaller trail races than the big road marathons. The overall race experience is much more satisfying!

Is the 50K a trail run?  If so, how much trail running do you do? 

Assuming the marathon training has been similar to the 50K terrain, you've done enough to have a good day.  There are a lot of ultra runners who run 50/100 mile events on back-to-back weekends, so having 3 weeks between the marathon and 50K shouldn't be a big issue (although...the strategy for back-to-back events is different than a single marathon)