You can always increase intensity...and it will always work - sooner or later the chooks come home to roost!
With fitness more effort (and/or time) is needed to expend energy (calorie burn).
Don't underestimate variation in your exercises (both different activities and intensity).
Not every day is/was a full "flat out" day for me.
I found it useful to have an "easy" day (well - an easier day) followed by a normal one and then a heavy day (really pushing it) and then back to easy (easing up from the previous tough day) etc.
I also used that pattern when exercising. I always started slow and gradually worked my way up in intensity.
I did this for three reasons. Firstly, the slow start was my warm up and helped me get my rhythm and technique right.
Secondly, you finish with your metabolism really revving and lastly - it felt really good when you finally stopped!
Intensity is created by putting in more effort and/or more time.
You have to put in more drive and/or prioritise your hours.
Music can help supply the energetic drive when one is feeling at low ebb.
For me songs with great loud driving beats like the Angels' Take a Long Line or inspirational songs like Bowie's "Heroes" can get my legs pumping until I'm powering up a hill and climbing to the top rather than limping to a stop!
Some days when the energy lagged and intensity just wasn't there I would plod along at a steady pace but stay out a longer time.
I would end up burning the same amount of calories but it would take me longer - at least that way I didn't burn myself out but did the work.
The source of all intensity is founded in attitude.
It always get backs to the brain where character is forged.
It is character that begats the right attitude and supplies the ultimate drive.