A few ideas:
Say there are 30 problems in a day’s work. Put a sticker after every 10th problem (or 5th or whatever you want) and have her get up and do jumping jacks when she gets to the sticker so she gets a quick brain break. Or set a timer and have her try to get to the sticker before the timer dings. For every timer she beats, there could be a little reward: sticker, a chocolate chip, or something like a coloring page, MagnaTile or Lego, then at the end of the lesson she can have a few minutes to play with the toys or color.
Have her use a wobble chair or yoga ball while doing math so her body can be moving while she works.
Split up the work into a couple of sessions per day. Does Math Mammoth have any sort of warm-up exercises like flashcards, speed drills, and skip counting? You could have her do that sort of work first, then move to other subjects, then come back to math for the main lesson.
Spread a lesson over 2 days.
Set a timer for the day’s lesson. Keep track of how many days she beats the lesson. If she can go a certain amount of time without going over the timer, you could make cookies or buy a new book. Usually I don’t like rewards, but she needs to see/feel the benefit of finishing on time in a way that is enticing to her. These reward-based ideas would be temporary–maybe for a month or two until it becomes a habit.
Make Friday a day for math games. Some good books are Games for Math, Math Detectives, Challenge Math, or RightStart has a book of math games. You can buy it in a set with all the cards needed. I don’t like their program as a main curriculum, but my kids enjoy playing the games after school time. 🙂 Anyway, a slight change in the routine can help a lot!