I'm not trying to depress you by pointing out that you will simply never achieve the level of performance that it takes to compete at the highest level of cycling, but that is likely one implication of this post. The following table shows approximately where you stack up in the realm of competitive cycling based on your watts per kilogram (W/kg) output at different intervals. As some of you may be familiar with an 8min test, to find your spot on the chart I suggest figuring your W/kg for 8min and moving up one or two lines on the 5min column. After training and power testing for 3-4 years now and upgrading through the ranks as my ability, both in terms of power and racing tactics, has improved I can vouch that as I have gone from a clueless Cat5 to the cusp of making the jump to Cat2 the power profiles contained in this chart have been pretty spot on. In fact my power output while in racing condition last year places me precisely in the overlap between the Cat3 and Cat2 ranges. This is exactly where I find myself, able to consistently finish in the top 5 of Cat 3 races and unsure about my ability to hold on for dear life with the Cat 1/2 crowd.
It is important to take into consideration the timing and conditions of your power profile before taking your "ranking" on this chart to heart. While my power at race fitness places me in the range I shared above, my current numbers following several weeks of rest and low intensity riding after a fall of grinding, muddy CX races are not going to be a good reflection of overall potential.
Several members of Team Indiebike p/b Angie's List have just recently "tested in" at the beginning of their early season training. Whatever those numbers are remember this is the baseline. As you add more intensity into your workouts and spend more time at and above threshold that W/kg number will creep up closer to its true limit. Also, as some of those extra winter kilos melt away the effect will be even greater. Depending on how your body responds to training you can try adding roughly 5-10% to your baseline power test now to get a rough estimate of where you may be once racing begins in a couple of months.
So you may never product 6.0 W/kg (410w for a 150lb rider) for 30 minutes and be able to repeat that multiple times in a four hour span. Most of us wont. Even so, knowledge is power so take this information for what it's worth and make the most of your training to find your limits and push beyond them.