some thoughts....
Just some thoughts....
Have you been active and just now are taking up cycling or are you coming from the ole couch-potato phase? If you were previously sedentary - it could simply be that you've taken on a lot all at once. If you were already active, cycling uses muscles that other activities don't generally use the same way (unless you're prone to walking up & down the stairs sideways!
)
Is your neighborhood hilly?
Are you getting enough protein and/or iron?
Have you had your bike fit checked? If everything is in the "wrong place for you", you'll be wasting energy, using smaller muscles when you could be using larger ones...
Are you pushing too hard? (i.e. are you using inappropriate/late shifting - you should be shifting sooner rather than later, especially if you've got any hills...)
You may want to consider "road" tires ("slicks") rather than combo on/off road tires for your mtb if you are riding on pavement; the extra texture on the combo/mtb tires can make you work harder than you need to be (I have a mountain bike as well, which I use for both road and trail/mtb riding - just switch out the tires for whatever riding I'll be doing that day!)
All who joy would win must share it -- happiness was born a twin. (Byron)