In reply to Donnie:
I'm in a pretty small club - total membership about 120, but lots of 40, 50, 60, even a few 70+
We've got I think 5 guys who have run sub 18 recently - 1 mid 30s, 2 late 30s, 1 early 40s, and 1 superfit late 50s.
I've just turned 50, been running at about 40-70 mpw for the last 2 or 3 years, and I've gone sub 19 about 5 or 6 times, pb 18:45. I like to think I could have gone sub 18 in my early 30s.
I'd still say that with no running background and just cycling 1 mile to work and 1 mile back again each day, then chances of OP doing this are very, very slim.
Given the timeframe, then sub 20 if he's got some natural ability would be an excellent achievement.
My guess, based on seeing other people start at parkrun is that he would do about 25 or 26 on his first attempt, if he trains regularly see a quite quick improvement obver 2 or 3 months to about 22 or 23, then plateau a bit, and with a considerable more effort push on to 20 or 21.