In reply to Roberttaylor:
It's generally considered easier to build endurance than speed, but you'll have more speed than you imagine - running in meadows isn't that fast, and a lot of it is just knowing how much pain you can endure.
Back when I was about 20, I did a bit of unstructured running. I could do a mile in 5.50 or less (no idea what, never did a proper test on a track) and did a marathon in 3.45 (so 8m30 a mile).
10 years later, I joined a running club, and did a lot of training, including proper speed work though admittedly not many short reps. My best mile was 5.20, but I did a marathon pretty comfortably in 2h59 (so 6m50 a mile). I suspect I could have done a 5 minute mile if I trained for it specifically - and perhaps still could - but it would have been hard work. Equivalent to, say, a 2h45 marathon.