[YZF]

Supposedly this is the definition: "Zone 2 is defined as the highest metabolic output/work that you can sustain while keeping your lactate level below two millimole per liter.". Unless you can measure lactate level you just don't know what heart rate this corresponds to. The heuristic that's used is "able to have a conversation".

For a beginner a good tip is just to go a little bit easier than they think they should be going. Once you've built some sort of aerobic base your training should start including intervals and runs that push you harder.

There are two risks in pushing yourself too hard. Injury and over-training.

[taeric]

So fairly removed from heart rate? Can I assume I'm not going to hard if I recover rather quickly?

[YZF]

There should be some correlation between lactic acid levels and feeling sore. I honestly just don't worry about it. Just getting out there and doing something is better than sitting around. Listen to your body. If you're not recovering, or not sleeping properly or are too sore, dial back a little on either volume or intensity. My Garmin gives me an estimate of recovery time, you really have to work hard to stretch that recovery time into more than 48 hours (e.g. running a 10K race). You can vary your workouts between something that feels extremely easy to pushing yourself a little more.

EDIT: another thing I didn't mention is that in terms of injury you want to give your body plenty of time to build more distance. Most of your joints and other tissues take longer to adapt than cardiovascular. I think this is where people can get into potential injury situations by ramping up too fast. I personally also try to run on soft surfaces (trails etc.) since I find hard surfaces (roads) a lot more punishing.