Johnson replaced Brian Tinnion in September, and lead the Robins from the relegation zone to a play-off challenge.
They won five of their six fames in April, scoring 13 times and letting in just four.
He told Sky Sports News: "It's taken time but I had to do what I thought was right and we eventually started winning games and getting on our way.
"Unfortunately it's just too late for the play-offs. But it bodes well for next season, when hopefully we can continue this kind of run."