I see this post is two months old, but
How old is she?
A rule of thumb for walking puppies is: 5 minutes per months of age allowed twice a day of forced exercise. Forced is anything on leash, on a hard surface in a straight line or compelled to keep up to you. Obviously by a year old an hour is permissable.
The rule also says free play off leash on a soft surface is unlimited as puppy can change speed, direction, even lie down and rest when she wants.
The goal is to avoid repetitive strain on young joints that are not yet solid.
I have a Lab, I followed this loosely. With a long backed breed I would be more careful, does she look more Doxie or Schaunzer?
I do note that you took her from 20 minutes to 50 in one fell swoop. An athlete building up endurance/distance would not more than double their exercise all at once, they'd gradually work up. You should do the same with your pooch.
For athletic breeds like my Lab exercise alone is not enough. Many need the mental work of obedience or rally or agility classes as well. Both Doxies and Schaunzers are smart little dogs, do you think one of these would appeal to her and you? It would complement the exercise, not replace it.