Base your measurements on time then, not miles. No matter what you're riding, if you ride "sorta hard" for an hour, you've ridden "sorta hard" for an hour. You might not go as far on the mtb but it's still the same time and effort, sort of.
I use time and heart rate (avg, peak, and total calories - which really is "how much relative work did I do compared to the other rides") to measure my efforts. This way I can sort of related efforts on the tandem, mtb, road bike, and indoors.
cdr
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"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson