Interval Training

Background: My last competition was in July. For August, I maintained an aerobic/fitness training schedule. September brought news of a major relocation, thus only 1-2x/wk practices. October brought me back to the midwest, with only a few trips to the pool. November I committed to swimming 2-3 workouts a week, and found myself sore. Here it is nearly Christmas, and I've managed 2x/wk swims of 2200-2400 yards/swim, without pain, split pretty much evenly between drills and swimming, with a couple of hundred kicking thrown in. My research has revealed no local masters' teams (in Fort Wayne), and despite my daughter's previous USS coach's enthusiasm to count me in his ranks, I have not found an age-group swimmers' team that would welcome me. Hence, I need to train on my own. Questions: How do I establish interval training that will prepare me to compete once again? Do I take best times and add x% and do x number of repeats? And, if so, with what amount of rest in between? My goal has to be to stay in shape until things settle down and get back into a serious training mode at the turn of the new year. Thanks for all suggestions!