30 miles even at a walking pace is still decent, though with no terrain it is not helping the fact. When i first bought my bike after not riding for about 8 years I thought doing 10 miles was awesome. That lasted about 3 rides.

Then I went up to 15-20 mile rides and at a harder pace (trying to pace faster than the 10 milers)

Once I got bored with that I moved up the scale to 30-40-50 miler rides trying to push the pace a little each time until about 50 miles. Then I was trying to ride at a consistent pace knowing I could be doing another 50