Last year, I rode 1-2 times/week, not much this year to some injuries (non bike related) and heat.
I measured distance by using paths with known distances for the most part or using something like mapmyrun.com
Speed - Not sure, but went out with a friend once and we were doing 12mph or so, she had something to calculate speed.
I have a road bike so riding off road is not a good thing
I used paved paths for the most part, sometimes short distances on gravel/unpaved.
My tip would be that riding a mountain bike will be a completely different experience than a road bike. You will be working harder to go shorter distances than someone on a road bike so it isn't a good comparison. I'd say just go out and have fun.