Keir, what sort of running machine did you get? I used them for a few years and found two things:
1) If you are to do a decent pace you need a machine with a long bed. A lot of machines you find in hotels and home gyms have very short beds and the advertised top speeds are belt speeds but require your cadence to be extremely high.
2) As DS2 says, you need some rough surface to strengthen the ankles. I got into a vicious circle where I had ankle trouble so used the machines to keep fit but whenever I raced on the country it damaged the ankle again. In the end I had to go back to the country and build up gently.
Just looked at amazon for P&D, can buy new for £11 or I can trade in my copy for £1.75. Lowest second hand price is £8.05 + delivery so there is a margin somewhere.
JD3, that is a big second week: 50 miles after a week off is going to stress you r body even at low HR. Hold back for a few more weeks. Half marathon plans come in three flavours:
1) Train for a marathon and put in some speed work.
2) Run a lot of half marathons in races and training and just get good at them.
3) Do a lot of training at shorter and faster pace and a few long runs and hang on.
The half is where a lot of people meet up so the long boys coming down a distance and the short boys going up one.