Nets already focusing on fixing two of last season’s weaknesses

Kenny AtkinsonNets players may have been working out informally for weeks, but Saturday marked the official start of training camp — one they expect to propel them to a serious playoff run.

They’re laying the groundwork for any postseason success before the preseason even starts. The Nets added offensive punch, but are trying to shore up Achilles’ heels such as defense and rebounding.

“We haven’t changed a lot, but we’ve tweaked some things, so that’s always a challenge,” coach Kenny Atkinson said. “But [a] lot of teaching. [It was] good.”

It was a long first day, with the most attention paid to those two weaknesses. The longest session — a full 20 minutes, a source told The Post — was spent…