It's a bit of a fine line, but the best way to gauge whether you're doing too much is simply your level of enjoyment. You should be looking forward to getting out for your daily run more often than not. We all have occasional days where we'd rather stay home on the couch, but if running consistently becomes a struggle and a grind, rather than something you look forward to, you're probably doing too much.
Long runs: Long runs (about 1.5 times longer than your usual daily run, up to 2 hours or so) can help accelerate the base building process. Long runs are very helpful in building the capillaries, the mini blood vessels that actually deliver oxygen to your working muscles. The more capillaries you have per muscle fiber, the more oxygen you can get to that fiber and the more work it can perform without fatiguing. Long runs also help you burn fat, store more glycogen (your muscles' preferred energy source) and are great for keeping your body weight down.
Tempo runs: Tempo runs are runs of about 20-40 minutes at a comfortably fast pace. These runs improve your ability to maintain a fast steady pace over time. Be careful not to turn these runs into a de facto time trial, trying to run a set distance as fast as you can. Run hard but relaxed, don't strain, and try to run with good form and light, quick strides. If you start to struggle, slow down a bit or cut the run short.
Speed work: Some runners will be surprised at the concept of doing speed work while building a base, and I certainly don't recommend doing a lot of high intensity interval training during the base period. However, as Lydiard said, it's a good idea to work on your speed all year round. During the base phase, your speed work should include a series of short sprints with full recovery (some call them strides, striders or accelerations). This lets you keep in touch with faster running without the strain of intense interval training.
Strength and flexibility: The base period is a great time to work on other elements of being a better athlete, such flexibility, muscular strength and core training. Stronger muscles make a faster runner by allowing you to generate more force with each stride, plus a solid core and flexibility help you prevent injury. You don't need to go overboard on this sort of training, but a twice weekly routine focusing on general strength and flexibility can make you a better athlete.
Other than the occasional striders or tempo run, your daily running should be a comfortable pace at which you can easily maintain a conversation. Base training isn't the time for beating yourself up with brutal training, it's the time for gradually building your aerobic fitness in preparation for the race specific training to come.
Here's an example of how you could structure a typical base training week:
Monday: Rest or easy aerobic run.
Tuesday: Easy aerobic run with 4-5 acceleration runs of 60-100m
Wednesday: Easy aerobic run, followed by a strength and flexibility program.
Thursday: Tempo run of 20-40 minutes with an appropriate warm up and cool down.
Friday: Easy aerobic run with 4-5 acceleration runs of 60-100m.
Saturday: Easy aerobic run followed by a strength and flexibility program.
Sunday: Long run, about 1.5 times your average daily distance at a comfortable pace.