I just finished 7 DCs to get that main prize chest (or whatever it's called). Took me a long time to get it finally. Out of all the decent prizes, I got 50 forge points. Sorry but I was pissed. The game just gives you FP for free!
Congrats on completing your first Challenger's Chest. You did just win 2 days of free hourly FPs all at once.
Keep working on your city, so you get the 7-day Challenger's Chest every 7 days, then getting 50 FPs won't sting as much. Look at the tasks you can't complete easily and use those as a guide on where to focus your improvement. Every task can be mastered by building up your city or your skills in some meaningful way. Keep plugging away. It'll take a while to get there, but that's what will give you a strong, well rounded city.
Be aware, the requirements for each DC task increases with every age. They'll take more goods, coins, supplies, and FPs to complete. If you can't complete most DC tasks with ease in your current age, you're not ready to move up in age. Slow down, get strong and let DC be your guide.
Pretty soon you'll rather have 50 FPs then another Wishing Well or Rogue Hideout that will just sit in inventory. At least the FPs can be put to immediate use. I do remember the frustration starting out when I couldn't get a Rogue Hideout no matter how hard I tried or how many DCs I completed. Now that I have what I need, they don't have quite the same value.