Last Fall I started cycling after a few decades away from it. I remember doing 20 miles in "X" amount of time. Went to work the next day and let all the athlete's at work I more or less "killed it" on the bike path. Talking like I had swam the English Channel. Later, a co-worker informed me I had averaged about 12 miles an hour.
This did not stop me from making a fool out of myself (again) the next month by annoucing my 33 minute finish time for a 5K.

The good news is I have improved a fair bit since then and it has been rewarding.