You can't go off your time in fell races either. The times are all going to be down at the moment as the weather has been so wet. You could quite easily lose 5 mins per hour dependent on the terrain.
If you want a regular check of your fitness, find a 4 - 6 mile route you can tempo every now and again on road. You will have some consistency to make a comparison with then.
Chris was on the right track I thought, but it does seem like you have that covered.
Feroglobin
http://www.vitabiotics.com/Feroglobi...d_proinfo.aspx
is a decent supplement to take.
In general, I have to say that when I train fast I run faster and I've had a lack of tempo in the last year or so and done much more longer slower stuff. That means I can get round a 2 - 3 hour run fine, but I was in bits after a 5K trail a few weeks ago that was 22:15 and nearly 2 minutes of my best time.
So having missed Duddon, I've decided to get some more tempo in for a few weeks and see how it goes, then try and get a better balance.