You built your base. Now it's time to incorporate hills into your training. If you live near hills, that's perfect. Start to do your shorter runs in the hillier parts of town.
During the next two months, transition your longer runs to the hills as well. If you live in the flatlands like me, dedicate one of your shorter runs mid-week to hill repeats on a treadmill. You can select a hill program on the treadmill or do it manually and control the hills yourself.
Start with one minute at 3 to 5 percent and follow with at least two to three minutes running at 0 percent to catch your breath. Alternate one minute on a hill with 0 percent grade for the rest of the run. Every two weeks add 30 seconds to the interval until you reach three minutes (e.g. 1:00, 1:30, 2:00?).
If you have one hill outside (or a parking ramp), you can run it repeatedly as well, making sure you have adequate recovery time in between each hill repeat. If the hill is short (under one minute to climb), start with six hill repeats and add one each week. If it is longer (more than a minute to climb), start with four repeats and add one each week.
There are two ways to tackle hills. One is to use the hill to make you stronger. This strategy is great for training. That is, you run up the hill hard, focusing on taking short, quick steps and pumping your arms.
The second is to make friends with the hills. This is a great strategy for race day, but should be practiced in training too. Marathoning is all about energy management. If you race hard up the hills on marathon day, you're going to race through your energy stores and fatigue quickly. It's better to take the hills efficiently and use the hill to make you run faster. Think like a cyclist and change gears going up the hill. In other words, shorten your stride, and maintain the same effort level (breathing, heart rate) as you have going into the hill. This means slowing down. Let the other runners pass you, it's all part of the plan.
When you reach the top, you'll have the energy to excel, use gravity and let the hill pull you down. Open your stride at the top, relax, lean gently forward and let it pull you down. Trust me--hills are a lot more fun when you have a good strategy. And this one not only conserves energy, but it can improve your performance too.
Good luck!