Most runners and walkers have the occasional problem with motivation. Often the cause is simply that you are following the same pace day in day out and have gone stale. So the solution is obvious.
You need to spice up your routine. Try sessions of differing distances. Vary your pace -- try running or striding out for two minutes and then take it easy for two minutes. Repeat four to six times after a 10-minute warm up and finish with a cool down.
Run your normal route in the opposite direction. Head for the hills or a beach for your weekend session. Run with a group or a pal when you can. Time flies when you're catching up with the gossip!