First, congratulaitons on losing 16kg! That's fantastic!
The trouble with places like Curves is that it does not allow you to improve beyond the initial plateau; know that it is not just you, but many others who have experienced the very same effect!
The body responds to demands- if you exercise in the same way for months on end, your body adapts to those demands within weeks or months (as it did when you initially saw results) and then stops improving once the activity is no longer a challenge. The key here is change- that'll keep it interesting, far from boring and a challenge! You'll need a more diverse exercise program to continue to see results.
On running; there are ways to gradually introduce running into your routine 2 -3 days per week. Start off with small goals, and alternate between running and walking, even if it is just to the end of your block. You can jog and/ or walk at a comfortable pace and progress from there.
I hope this helps! If you've got more questions, don't hesitate to contact me, or the personal training team!