One of my favorite rainy day workouts are indoor intervals. It's a great way to stay in shape despite the weather outside, and is one of the best ways to not get bored on the treadmill, not to mention a perfect repertoire for your race training! I love using my new TIMEX80 vintage watch - it's a simplistic design that helps me track my splits, and is perfect for interval training.
I typically like to do intervals for about 30 minutes, and cool down for 5 minutes afterward, but you can do interval training for a full hour if you need to get in long distance speed work for your marathon training. I like to start out at a comfortable pace at 6.5 mph - you can adjust based on what your comfort zone is, but try to stay in the range of 5.0 mph - 8.0 mph. For the first 5 minutes, warm up to your comfort zone, after five minutes, increase your speed by 0.5 mph, and maintain that increase for 2.5 minutes. After 2.5 minutes, decrease your speed by 0.5 mph, and maintain your comfort zone speed for 2.5 minutes. After 10 minutes, increase your speed by 1.0 mph, and maintain that increase for 2.0 minutes. After 2.0 minutes, decrease your speed by 1.0 mph, and again, maintain your comfort zone speed for 2.0 minutes. Continue this pattern until you reach a 30 second interval, and 30 second cool down, then repeat the sets for your desired time. I've included a chart below to give you a visual of the interval levels.