The third Sport Relief kicks off in March this year and England's Jermain Defoe has helped get it started.
The Tottenham player took some time out from training to explain what's going on this time, and hand out some top fitness tips to Newsround's Gavin.
And he might be in need of them too, as the main event of Sport Relief is a run of either one, three or six miles.
Jermain said: "It's about having fun. Playing sport, hopefully football, and raising loads of money for kids."
He added: "To be involved is great for me. It's for kids, and I remember when I was a young lad on the streets playing football.
"It's a special thing, so to be involved is special for me."
Jermain told Gavin that stretching before running is really important, and added that playing football regularly should keep people fit enough to complete the runs.
And Jermain also persuaded Gavin to practice sprinting and star jumps.
After all that work Gavin was convinced he could take on Jermain on the pitch, and took up a position in goal, ready to pull off a miracle penalty save...
Which he didn't quite manage. Gavin did go the right way, but wasn't quick enough to get his hands to Jermain's powerful spot kick.
So far Sport Relief has raised more than £20m for charity, and organisers hope this year will be another success.