This is the most important screen in all of Xert in my humble opinion.
Suggestion #1
It shouldn’t be a pop-up. Integrate it as part of the Training Advisor page.
Suggestion #2
You have “Estimated Weekly Training” for the current chosen Improvement Rate. However, since I prefer to think in terms of XSS (and I think you have a blog post that implies that you do too) could you please include an estimate of the Weekly XSS at the chosen Improvement Rate?
Suggestion #3
Instead of forcing the user to click through each of the Improvement Rates to see the resulting projections and estimated weekly training time/XSS, please just pre-calculate it for all the rates and show all the projections in a table. Then let the user choose from the table without the need for pressing an Update button.
Suggestion #4
Do away with the star-based system for workout Difficulty rating and just use the numerical Difficulty score everywhere. I feel this will make it easier to use the Freshness Feedback scale. At the moment, I find myself iterating multiple times between tweaking the Freshness Feedback, and seeing what workouts the Training Advisor suggests.
Suggestion #5
For reference, show your current Improvement Rate based on the last week and last month of activities. This will I think help ground the meaning of the different Improvement Rate levels in the user’s mind. I also think it will help me choose the appropriate Improvement Rate for a rest period in the context of my recent training.