I Love Working Out

This week I have been trying a new tact with my workout. I've been noticing that I am not getting that endorphin rush after and I miss it lol. I had someone tell me that it means my body is getting used to the current level and in order to get to that rush I have to push harder. So I have been taking a more "circuit training" mindset. Instead of walking my 20 on the treadmill and then doing the other parts, I am doing 5 minute bursts...then do my weights, balancing, crunches...back to 5...and so on. I find that it is consistently keeping my heart rate higher...but is this a good way or would the solid stretch be better? (I also do and morning and evening walk, but this is my midday workout, which I need to get to lol.)
Any advice?:)