You can do the vast majority of your training off track, especially if you're focusing the racing cross country or roads. Just use do fartleks with perceived exertion and time.
This isn't, however, a complete solution to your problem.
You mention a lack of motivation to do hard workouts on the track. You're likely to lack the same motivation on the roads. You just won't know that you're not going as fast. This isn't always a bad thing. Sometimes the knowledge that you're not hitting your paces on the track is so demoralizing that you end up not getting in much of a workout at all, whereas if you're just going by perceived exertion, you might be going to slow, but at least you'll get in some solid volume. Also, going a bit slower in your workouts for much of the year is a good way to stay relatively fit, avoid injury, and prevent an early peak.
Ultimately, however, if you really want to race well, you're going to have to find a way to push yourself harder during the last 4-6 weeks before your peak races. You need that really hard finishing work to maximize your physiological and psychological fitness. (A big piece of the puzzle is getting familiar with pain and developing the motivation to push yourself through it.) You don't necessarily have to do this work on the track if you won't be racing on the track, but you have to figure out some way to get that work done.
I'd suggest 2 options. The first is to get training partners. Almost everyone runs harder in a group. By taking turns leading (or just following the strongest guy), you can get pulled to times you wouldn't hit alone.
The second option is to use lots of tune up races. This only really works if you're doing shorter races. If your peak race is a 5k, though, you can do 4 5k road races in the 6 weeks leading up to your goal race, and that should get you pretty sharp.