I started out doing 3 miles...and it took a while. I just stayed there for a long time.
I think those mountain bike miles are tougher miles than road bike miles, but that's just me.
I got a new bike about 3 weeks ago, and increased my miles slowly* so that I wouldn't get any aches and pains that would put me off biking. Then I decreased my miles accidently because my odometer reset to kilometers and I didn't catch on. But today I was able to bump up the gearing I usually ride in and added 1.2 mph to my speed and was not at all tired after 8 miles. But, I'm forcing myself to slowly add the miles. They say to increase your distance by 10%, but I'm not sure what time frame that's in. So I'm doing it twice a week.
In the beginning, that meant only adding 1/3 mile to my distance twice a week, but what's cool about that is you can pay close attention to any discomfort you feel and fix the problem without actually hurting* yourself. If you're at 6 miles and you feel a little neck twinge, it's a good thing you didn't push it, go 12 miles and end up with a sore neck for a week and not be able to ride!
At least, this is my opinion about it! But my nature is to let things progress slowly.
P.S. Wise BF people say it's good to get on the bike 5-6 days a week, even if you only go around the block. I think they're right... somehow it helps the body get adjusted to it. My bottom's never been sore, even on a new leather saddle.
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