You sacrifice some of the specificity of your training, and eventually your pace judgment in races may suffer.
Think about getting back on the roads once a week to sharpen your pace judgment. If you think you can't tolerate the hardness of pavement, try a rubberized synthetic track. Any surface that is smooth, firm, uniform and measurable will do.
Here are two suggested workouts:
1. Locate an accurately measured one-mile stretch of road, or a track. After warming up as much as you would before a race, run a mile at your current race pace. Walk or jog a few minutes to recover, then do another mile (reverse direction if your measured mile is point-to-point). Again, aim for race pace. Repeat a third time if you're not too tired. Conclude the workout with an easy cooldown.
2. For variations on this workout, try running measured distances of less than a mile, or practice a variety of paces (10-mile race pace for the first mile, 10K pace for the second mile, 5K pace for the third mile).
A word of caution: These are challenging workouts, so treat them as quality days in your training schedule. Be sure to allow sufficient recovery before running hard again.