Greg Plitt talks insanely passionately about the reasons people start training regimes then quit. I agree 100% with what he says (even though I think he should probably slow down and breathe a bit more during the delivery ;)
1. Find a training partner
2. Track your results
3. Find a future event that makes today reliable and accountable e.g. a race, wedding etc.
4. “Find a new atmosphere”
5. Have a plan of attack – it’s a marathon not a sprint