JB, Thats good news. Try to stick at the lower intensity (this is where the big initial aerobic gains are to be hadd) even though you think yo may be going too slow. Remember, miles and pace are incidental. The key factors in this type of training are time and HR.
Do you do do all your running on a lunchtime from work? I ask because this type of plan will draw much more improvement if you can build up to 70mins.
You've read the Hadd document so you will be aware of the toothpaste analogy which I think sums it up perfectly. You are squeezing right at the bottom of the tube at the moment. Get right into the bottom corners and make sure you don't leave any toothpaste behind.
It's so easy to worry about the races you have coming up. Obviously you will be running at a much higher intensity than you have trained at for a while. But surprisingly you will be amazed at how comfy you feel. It may take a little while longer than normal to get the legs going, but you will be able to achieve reasonable results despite the lack of speed. Plus you will generally feel better than normal following the race, maybe even to feeling able to keep going after the finish.
Remember - forget about miles and mileage. Think about time and HR. I generally used to run outandback courses, as this made it easier to achieve the required time. Also, take a split when you turn, and then try and keep at the same HR. There will inevitably be cardiac drift, but this type of training will eventually cut that to a minimum.