Some good advice given already. I'll add one thing that's worked for me recently... and it's an accidental find.
I've started going to the gym, and a favourite activity is the rowing machine - to get the work out, a bit of cross training, and good cardio. I've been doing a fairly hard 5000m programme.... that takes me about 24 minutes. There's a figure (who knows how accurate?) of calories/hr.... and I try to keep the figure above 750 for the first 4000m then above 900 for the rest. This takes a lot of perserverence as I stare at these figures in front of me.
I've found now that when going up a long steep hill (part of one of my favourite runs), I've fallen into this mental routine, imagining myself perservering on that machine... and it just keeps me going.
Maybe the improved leg strength of using the gym helps, but it does feel like a mental thing.