I'm going to give you a number to work with in your training and here it is: 20%.
20% is the limit and the goal. 20% is how much of your training should intimidate you. Anything more than that, you are going to get injured, sick or burnt out; anything less, you are not training hard enough. So 20%.
How you do the math from there is up to you, but monitoring your heart rate is a solid choice. I'm not going to try to convince you of the merits of heart rate training as a guiding light in your fitness pursuits, but I do think that you ignore it or choose other methods at your own peril. Heart rate is a solid measure of intensity and recovery, especially when applied to total training minutes over the course of a cycle.
A good heart rate monitor (I use Polar products), a good heart rate variability app (I use both Polar's in-system analysis and the HRV4T app on my iPhone), and a sense of what all the numbers mean can go a long way toward your success. There might be a learning curve to start, but come on--what doesn't have a learning curve? There's a reasons why #fullcontactlearning and #dobetter are two of my most used hashtags.
The benefit of 20% is that most of the science supports the idea that it's a good ceiling on hard efforts. As far back as Arthur Lydiard and as recently as Matt Fitzgerald, running coaches have been extolling the virtues of limiting intensity in training for the best possible outcomes.
It doesn't just apply to running, either; you can apply this unilaterally in your training. Personally, I apply it to my strength training, my martial arts sessions, cycling, inline skating, and whatever other sport I'm using for training or cross training or competitive purposes. To be honest, my most intense efforts probably come in closer to 15%, but it goes as high as 20% at times. Regardless, it works. Get in that range and you might be surprised at the results, particularly if you haven't been paying attention up to this point.
If you don't know what percentage of your training is high and low intensity or even what those things mean, you have some research ahead of you. It will be time well-spent, though; a little full contact learning will ensure you do better.