I wouldn't worry about going too fast at the moment. Your pace sounds fine. It's very similar to my pace for tempo runs and I have been around these paces since I started. Everyone is different. If it feels comfortable then just do it. Your body will tell you if it is too much. Just listen to what your body is saying and don't worry at the moment about what the watch/plan is saying. As I said, if you are always feeling like you have nothing left in the tank at the end and going out for your next session just seems like impossible as you are so fatigued then then is the time to slow down and rethink (and obvioulsy if you are getting hurt)
I certainly agree however that there is a pace that feels too slow, where it hurts more to run at that pace (like my long run pace is around 11/11:30 and I feel that hurts me a bit on the knees) When you start going for much longer and pushing the time, you will find that you slow down naturally anyway and just have to resist the rush to start off too quickly when you feel fresh! 
I don't know how fit you are -you certainly sound in good shape, but just be careful about going out for so long so early in your plan, especially if running on concrete. Though you might be fit, your legs might not be used to the battering of the hard surface so let the weakest part of you get stronger first. (Very frustrating, I know but less frustrating than weeks out due to shin splints)!
I don't know this plan, but it sounds quite hardcore, like it's for someone gettting prepped for the marathon quickly. How long is the plan? Is it a total beginner plan? I would guard against embarking on a marathon plan without a good base. Doing the half has made me realize what a commitment even that is, I don't think that I am ready to commit to a marathon yet, and even after getting much stronger cardio and muscular wise, I am feeling the fatigue with 5 weeks till the race to go.
Hope this helps, let us know how you get on - just enjoy it and you will soon be hooked.
Oh, and I am the biggest tracker of pace, time, routes in the world, but I don't have a HRM, some people swear by them, for me it's just another thing to get stressed over and to add to my over analysis. Since I have gone back to basics and started listening to my body, my running and the enjoyment of it has improved a hundred fold