I ran two 5K time trials over the past month. The first was run on grass with rolling hills at a local park, using my Garmin for measurement (thus, only accurate to within about plus or minus 50 feet).
The second was yesterday on the track, by myself, and under VERY windy conditions. The wind gusts were strong enough at times to nearly knock me out of my lane, and I was having trouble running in a straight line. The point is that my time was probably 15 to 30 seconds slower than under ideal conditions, but as I did not run a simulataneous control case on a track with no wind, the differential from "ideal" is strictly an estimate.
With that said, the cross country time trial at the local park was run in 21:32, or about 6:56 per mile, and the track time trial from yesterday was 20:24 (6:34 per mile). Given the differential between grass trails with hills and the track, I would still say that the track time trial was a better effort overall. My goal was actually sub-20, but after a 6:42 first mile, and 10:25 at the halfway point, I knew it probably would not happen. Mile 2 came at 13:22 (about 6:40), then I managed to pick up the pace to about 6:20 per mile for the final 1.107 miles. This was a 90 to 95% effort, in which I felt quite winded, and probably could not have run too much faster.
I still have a hopeful season's goal of sub-19 by early December, but that is going to be very tough. This means at least one speed workout per week, and possibly two for the next 5 weeks.
Once again, I'm having thoughts of retiring from competitive running. I'm not sure why I need to go out and suffer at 50 years of age.