Hi Karna! That's a great question! There are two types of x-training workouts that will help with running speed. One is cycling, with speed intervals. If you do not have a bike or don't like cycling outside, SPIN class is a good way to get this exercise in. Classes usually include speed intervals. Another workout is strength training, aka lifting weights. Not only will this help produce more speed, but also helps with injury prevention. HIIT workouts are good, but I would avoid those during training (or high mileage weeks) and opt for sets & repetitions.
Check out these 2 amazing articles I found, for more information:
Any high intensity interval training that challenge your Type lla muscle fibers (just think fast-twitch) will help you to become a more powerful runner overall and it’s great for cross-training!