In reply to pepperpot:
I broadly agree with what is being said: listen to your body.
BUT don't overlook the fact that tools (inc. HR) can help those of us that are a bit deaf to their body.
I for one started running and let it rip. I got several niggling injuries. I then did a really good block of slow stuff, plodding along, walking the hills etc. and used HR as a meadure to stop me getting carried away.
This helped stop me going over my limits and the injuries went away. Since then I have thrown the HR away as I find it resticting (mentally). But in the mean-time I have learned to listen to my body which I was incapable of doing before!
Great work getting out there, you will get better!!