In reply to ColinVern:
It will be difficult to pinpoint, since many factors are subtly different outside compared to inside, but I have found it very difficult to match my top treadmill pace when outdoors. I don't use the treadmill any more, but when I did I could do 15 kph (6:24 mile pace) for 8k ie a 40 minute 10k pace, (but not quite for 10 k,dammit!), with patches of 1000m at 16 kph. Outside, despite often trying, I struggle to keep that up for more than a mile or so, in fact I don't think I have ever managed 16 kph (6 min mile pace, except once when gravity was on my side) even on nice, flat, soft, hard, whatever surfaces.
I wonder if it is a mental thing: the treadmill just keeps you on it, whereas outside it is that much easier to slack off, and you know it, so keeping up the pace seems harder and your body reacts.
Clearly, I don't know! But I hope your pain goes away.
Post edited at 18:30