Set yourself goals
Because they give you something to focus on, you can measure if you're making progress towards it and it can be very gratifying to accomplish them!
For example when running I'm using MiCoach from Adidas, though any performance monitoring gadget will do. It allows me to keep track of my progress and see how well I sticked to my workout.
It also keeps track of overall statistics and gives 'achievements' when you hit a certain milestone. But off course, you can use several metrics to keep you motivated:
- try to run longer,
- try to run faster,
- lift more weights,
- do more repetitions.
Just measuring your exercise and using it to set goals can be a great motivator, especially when you can't exercise in a group! But off course, this isn't suited for everyone. You need intrinsic motivation to keep this up. Otherwise, I agree with the rest: go run in groups!