I have the same problem - get home from work, check kids homework, other stuff, then riding time. My daily ride is either 10 or 15 miles depending on how much daylight left.

My 15 mile ride has two fairly good hills that take 10+minutes each to climb. They are not steep grades but rather extended in length and about 5% grade (maybe max about 7% on a few 50 yard parts). Sometimes the long one can have a headwind. Evaluating the varied terrain of the 15 mile ride, I don't really need to do intervals. The sustained climb/rest/second sustained climb give me natural intervals. The first climb comes at about mile 3 and lasts about 1.5 miles so I have a few miles to warm up. The second climb is at about mile 7 to mile 8. I then have about 6 miles of more rolling terrain until I get home.

My 10 mile ride has several good/short hills that require me to use my quads and get my heart rate up/then down as I complete the hill. I try and sustain the effort after these hills for 50 or so yards.

I find that this workout gives me natural intervals. If you ride a similar route I don't think you need to necesarily concentrate on intervals - you get them via the terrain. If your route is flat then you would need to use intervals to move your heartrate up and down. Swimming using intervals because - well- when you swim it's all flat. Training for basketball and footbal and baseball and hockey and soccer and lacrosse are similar to swimming - running on a flat field. For those sports, to build endurance requires intervals. But if your cycling takes in hills then I think you get natural intervals as long as you try and attack the hills/use them and your fight against gravity to improve your performance. If you just slip into your granny gear on your daily rides and ease up the hills you won't get the same benefit.