I'm trying MAF for 3-4 weeks again, see how it goes.
If you've not heard of this type of training before Google it, it's worth considering. It's basically running to heart rate (180 minus your age) all the time apart from racing. It's enough to improve yourself but not enough to damage you.
I ran 12 miles yesterday and only just kept to maf. 141bpm and 6:17 pace, road and slightly windy. I think my maf should be 148 (I'm 37) so 5+ for fitness. Yesterday my legs were tired after a 5k race on Saturday so I was struggling to keep my heart rate up. My heart was chilled out! But my legs hurting.