Came back from a short spa break yesterday. While I was there, not wanting to miss a run, I did W5D3 on a treadmill, with a cool down of a few pool lengths and it was no problem at all, almost easy in fact. Thought I had better consolidate the W5D3 with a normal outside run, in particular because on another forum people have said treadmill running is less effort and also because I did it at zero gradient and my normal run is quite hilly. And this morning, I just didn't manage the whole 20 mins. Should I do it again on Tuesday and count today as a fail or should I just move on to week 6 knowing I have managed to do it once? Is treadmill zero gradient cheating and have I failed?