I ride trails and have a few rides mapped out. One in particular I sometimes do multiple times; actually, it's two connecting trails, one 4 miles (so 8 out and back) and the other just over one mile (so 2 out and back). In the evening, I rarely have time to do this ten-mile run twice -- I'm slow -- but sometimes, I do have time to do the one-mile trail twice, so will add that for a total of 12 miles for that ride. Other times, I'll nearly finish the ten-mile run, but turn around at mile 9, go back to 8, and then turn around again just to add two miles to that day's ride.

I'll sometimes do this ten-mile run 3 or 4 times on a Saturday or Sunday, and even then, will sometimes double back (on the part of the trail I like the best!) to get in another couple miles.

So... you might want to pick a favorite ride, and see where you can double up or double back to add, say, 1 mile to the 10 mile run (doubling back a half-mile). Do that a couple of times, and then double back for a mile to make it to 12 miles, and so on.

If I'm going for miles (and I have been lately, to prepare for a ride I'm doing next month), I find it helps to ride a trail that is really really familiar to me, one I really like. The miles seem to go much faster when I know the trail very well.

And yes, using a lower gear should help on the hills. My tendency is to push harder rather than pushing more (fewer revolutions rather than more), but I'm old and my knees are *not* happy with that! I am now trying to ease up and spin more to save my poor old joints!

Good luck!