Why don't you suggest a challenge automatically? And one can opt out/choose individually if one wants.
Analogy to normal learning:
Imagine all the cards to learn Spanish would be in a list and as a learner, I would have to click through all the cards...
How many people would do that, and how many fewer than there are now (with your autosuggesting cards until you reach 20 - 50 - 100 cards)?
"Don't make me think" (Only on learning Spanish itself, please. Give me an auto-option and if one thinks one can decide better, give those people an option to do so. Now having to choose yourself is a default...)