AndrewD - I use a mixture of heart rate and paces. Mainly heart rate to make sure I don't go too fast and paces for the speedier stuff. That 220-age wouldn't work for me. When I did the proper test my max came out at 192 but the age calculation would give me a max of 177, so quite a big difference!
I will be jogging round parkrun with my daughter as I have 15 miles planned for Sunday.
Nicole - it sounds as if you need to give your body a proper break! Lurching from one injury to another will mean you never reach your full potential. One of the things that will give you most improvement is consistent training and your body doesn't seem to want to let you do that! Also, are you still aiming for a sub-4hr marathon? If you are, then that 18 miler you ran seemed to be too fast for a 9 minute mile marathon pace. Or at least it was at the very fast end of the pace range you should be running it at. I think you need to slow down!