I'm impressed, to maintain a pace considerably (and by considerably I mean an awful lot!) faster than your best over 5k, and probably faster than you can run a single mile for 20 miles.
It is, and reading back my post is probably a little too caustic if it's a genuine runner!
As a low 33 10k runner though, I've done 17 miles in just under 1:40 which was working moderately though not overly hard. This chap is saying he's done 20 with a 5k PB that's closer to my 10k than my 5k!
Good on him though for getting out there and running for that length of time-maximum I'd say he'd have run probably just over 10 miles- maybe 1 or 2 more.