One of the problems with Sprint races is that there is no standard bike distance. In my area I've seem distances from 10 miles to 18 miles for Sprint races. So if the race was only 10 miles, I would think you could really hammer! I would think that you'd want to ride at least half again as far for your normal training rides.
I like sprint races and do a fair number of them. Don't have a Power Meter so have no gauge for actual wattage generated. But I do track my rides with CycliStats. I've got a 25 mile loop that I ride all the time. It's all country rolling road so very safe and relaxing. As the Spring progresses and I get into Triathlon season I try to better my time from the previous ride. Obviously it's very weather/wind dependent, but the statistics show that my times are getting substantially faster this time of year. Also as you make changes to your bike such as aerobars, position changes or wheel covers, you can see the effect on your average ride speed when new rides are averaged in.
The idea that you want to hit a specific wattage doesn't make sense to me. It take time for your body to get stronger. Develop an aerobic base and than continually build on it. It might take a few seasons to generate the power that you want, but you'll eventually get there! Plus, training on a indoor trainer is essential during the off season. That way your maintaining what you worked so hard for all season. Otherwise, in the sprint your just playing catch up!