McCullum said there is a danger of over-doing it at training after having to spend days on end watching rainfall from hotel rooms.
"It's been a good opportunity today to have a decent training and spend some time in the nets and work on a few things."
"You're actually wanting to get a lot more done so I think sometimes you have to pull things back a little bit because everyone wants to over-train when you haven't been hitting balls or bowling too much because of the rain. You've got to be cautious of how much you do when games are coming up close by."
The Black Caps head into the match having won just one of their last 18 ODIs against Sri Lanka on their home soil, that victory coming nine years ago.
McCullum says they are being competitive but need to make the most of the key moments in each match.
"At the moment we're just sort of sitting there and they've taken the game away from us a couple of times. We really need to pick up those critical moments and run with them and try and win those critical moments. Hopefully that will put us in steed to win the game eventually."
The two sides meet at the SL Mahinda Rajapaksa International Cricket Stadium in Hambantota for the first time. Sri Lanka have played five matches at the ground since its first match last year winning three including victories over India and Australia.
With a small sample size it's hard to tell what could be a competitive total with India making 314 against the hosts earlier this year before being bundled out for 138 three days later.
"I've heard it's a big ground so that will bring the spinners into play a little bit more, weather permitting," McCullum said.
"It will be good to get up there and hopefully get a couple of dry days and a couple of decent days of cricket which we can get a couple of wins."
- nzherald.co.nz