My other half usually only does a mile or two to work everyday.
Longest ride she had done was 16 miles which we had done a couple of times. I booted her backside out on a 50 mile ride to Brighton and back a few weeks ago. Took it nice and slow and had frequent breaks, it killed her, but she did it.
If you can do 20s pretty often, 50 is only a bit further.
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