I'm no expert, Scott, but I'll share with you my thoughts. They might be helpful, since you and I might be similar runners.
I am 40 and have been running for about six years. For quite a few years now, my "basic" run is 4.5 miles. I usually run that at around 8:30-8:40/mile. Unless I am in a marathon buildup (I have run 3), I usually run about 25 miles in a week. In 2005, I added a once a week "tempo" run during which I ran 2.6 miles in approx. 18:40-19:00 (7:10-7:20). Up through 2006, my best 5K was 20:19.
In late 2006, I added a once per week day of intervals (mostly 800 meters in approx. 3:20 (6:42 pace)), and, in 2007, I changed my tempo route and I now go 3 miles in approx. 21:40 (7:13/mile); that route includes a gradual one-mile climb that takes about 7:30-7:40 (so, for the other 2 miles, I am running approx. 7 flat). Although I haven't done any marathons this year, I have been doing weekly 12-mile long runs at 8:50-9:00 pace.
This year, I have set several 5K PRs including my last 5K, run one month after my 40th birthday, in which I ran 18:44. I'm faster now than I've ever been in my life! I'm giving you a larger picture than you asked for, in part to explain the other things that are going on surrounding my tempo runs.
Tempo runs are basically designed, as I understand them, to increase your lactate threshhold. For a 5K, I don't think you need to run any more than 20 minutes at tempo to accomplish what you need to accomplish. What you did at lunch was great. Don't go slower, don't go faster. Keep doing that. If you want to run faster for the 5K, in my opinion you'd be better off adding some speed work/intervals to your training than in increasing the distance of your tempo run. When you train for a 10K, half marathon or marathon, then you can start increasing your tempo length/distance.
By the way, I couldn't run 7:00-7:15 for one hour during my training. But I do think I could run that pace if I tapered and raced it. I say that just for perspective because you said that you didn't think you could hold your pace today for a full hour. I couldn't have held my tempo run pace this morning for a full hour either. Just sayin....
Good luck!