I did not realise that they can come so far up only winning Challengers
You win 80-125 points for winning challengers. Janowicz has won three this year (a 90 and two 80's) and reached the final of another. The guys with similar records on the challenger circuit (winning 3+) do tend to be in the top 80 in the rankings. Most of Kuznetsov's points come from challenger titles, Goffin and Klizan won a few too.
Janowicz also qualified and reached R3 of Wimbledon (115 points including qualies), so not all of his points are from the challenger circuit, although they do make up a large patch of his points. But many players outside the top 50 play quite often on the challenger circuit. Ones like Janowicz are rare cases though - those on a breakthrough year. Him, Klizan, Goffin, Zemlja, Kuznetsov, etc will probably only play a few challengers next year now their ranking is so much higher that they can get into the main draw of almost every tournament.