Starting a new training plan that calls for a timed mile. My son paced me to a 6:19 mile on the track. It was waaaay easier for him (17 year old HS XC and Track kid) than it was for me. I didn't think I was gonna survive.
Oh, and he had just run 12X400 at track practice before that. Made me feel like I was 152 instead of just 52.
Consistency: It's only a virtue if you're not a screw-up.