As carbonfiberboy says, the two main aspects of training are volume and intensity. The volume helps develop your cardio-vascular system (new capillaries develop etc.), Intensity layered on top of a solid base will drive more noticeable improvements. A good reference are the books by Joe Friel. I fall into the over 50 crowd, so I have his Fast After 50 book that lays that out. The intensity can be hill repeats, or it can be surges on flat ground. There is more risk of injury with intensity, and recovery is needed between intense efforts. Start off with only a couple of intense sessions per week, and keep the remainder of your volume easy. Best of luck. Lots of effort can go into planning how long the hard bits are vice the recovery periods (30 sec/ 30 sec or 5 minutes / 3 minutes), but the basic idea is to work hard to force physical adaptations, and then recover so your body rebuilds. Start with what works for you, works with your terrain, etc.