When I got my first couple of challenge games, I thought for sure that I could just power through them and then get back to the "real" project. I now realize that was a naive belief. Playing a game with no plot or internal goals and unpolished mechanics is surprisingly exhausting. I wouldn't have thought so initially. I actually thought that even a bad game would be diverting compared to, say, staring at a wall for 20 hours.
I'm not sure I believe that any more.
So, I will alternate between challenge games and my regular steam library games. The exception is that I will try and play series of games back-to-back. I'll go down to the challenge list in the order the games were received, and down the library list in alphabetical order (but I'm willing to take requests, if any of my readers have a particular game they want to see).
I think this plan will lead me to complete my goal as quickly as possible.